fix(gen_help_html): handle delimiter, heading

Problem:
vimdoc grammar added new forms that are not handled in our HTML
generator. neovim/tree-sitter-vimdoc#134

Solution:
Update `gen_help_html.lua`.

Fixes neovim#29277

runtime/doc/usr_21.txt

@@ -303,21 +303,21 @@ use, and save this in a session.  Then you can go back to this layout whenever
 you want.
    For example, this is a nice layout to use:
 >
-	+----------------------------------------+
-	|		   VIM - main help file  |
-	|					 |
-	|Move around:  Use the cursor keys, or "h|
-	|help.txt================================|
-	|explorer   |				 |
-	|dir	    |~				 |
-	|dir	    |~				 |
-	|file	    |~				 |
-	|file	    |~				 |
-	|file	    |~				 |
-	|file	    |~				 |
-	|~/=========|[No File]===================|
-	|					 |
-	+----------------------------------------+
+        +----------------------------------------+
+        |                  VIM - main help file  |
+        |                                        |
+        |Move around:  Use the cursor keys, or "h|
+        |help.txt================================|
+        |explorer   |                            |
+        |dir        |~                           |
+        |dir        |~                           |
+        |file       |~                           |
+        |file       |~                           |
+        |file       |~                           |
+        |file       |~                           |
+        |~/=========|[No File]===================|
+        |                                        |
+        +----------------------------------------+
 <
 This has a help window at the top, so that you can read this text.  The narrow
 vertical window on the left contains a file explorer.  This is a Vim plugin
@@ -448,9 +448,9 @@ trust the files you are editing: >
 
 	:set nomodeline
 
-Use this format for the modeline:
+Use this format for the modeline: >
 
-	any-text vim:set {option}={value} ... : any-text ~
+	any-text vim:set {option}={value} ... : any-text
 
 The "any-text" indicates that you can put any text before and after the part
 that Vim will use.  This allows making it look like a comment, like what was
@@ -462,9 +462,9 @@ using something like "gvim:" will not work.
 typing the ":set" command, except that you need to insert a backslash before a
 colon (otherwise it would be seen as the end of the modeline).
 
-Another example:
+Another example: >
 
-	// vim:set textwidth=72 dir=c\:\tmp:  use c:\tmp here ~
+	// vim:set textwidth=72 dir=c\:\tmp:  use c:\tmp here
 
 There is an extra backslash before the first colon, so that it's included in
 the ":set" command.  The text after the second colon is ignored, thus a remark

scripts/gen_help_html.lua

@@ -490,7 +490,6 @@ local function visit_node(root, level, lang_tree, headings, opt, stats)
     or nil
   -- Parent kind (string).
   local parent = root:parent() and root:parent():type() or nil
-  local text = ''
   -- Gets leading whitespace of `node`.
   local function ws(node)
     node = node or root
@@ -508,6 +507,7 @@ local function visit_node(root, level, lang_tree, headings, opt, stats)
     return string.format('%s%s', ws_, vim.treesitter.get_node_text(node, opt.buf))
   end
 
+  local text = ''
   local trimmed ---@type string
   if root:named_child_count() == 0 or node_name == 'ERROR' then
     text = node_text()
@@ -537,12 +537,17 @@ local function visit_node(root, level, lang_tree, headings, opt, stats)
     if is_noise(text, stats.noise_lines) then
       return '' -- Discard common "noise" lines.
     end
-    -- Remove "===" and tags from ToC text.
-    local hname = (node_text():gsub('%-%-%-%-+', ''):gsub('%=%=%=%=+', ''):gsub('%*.*%*', ''))
+    -- Remove tags from ToC text.
+    local heading_node = first(root, 'heading')
+    local hname = trim(node_text(heading_node):gsub('%*.*%*', ''))
+    if not heading_node or hname == '' then
+      return '' -- Spurious "===" or "---" in the help doc.
+    end
+
     -- Use the first *tag* node as the heading anchor, if any.
-    local tagnode = first(root, 'tag')
+    local tagnode = first(heading_node, 'tag')
     -- Use the *tag* as the heading anchor id, if possible.
-    local tagname = tagnode and url_encode(node_text(tagnode:child(1), false))
+    local tagname = tagnode and url_encode(trim(node_text(tagnode:child(1), false)))
       or to_heading_tag(hname)
     if node_name == 'h1' or #headings == 0 then
       ---@type nvim.gen_help_html.heading
@@ -555,7 +560,9 @@ local function visit_node(root, level, lang_tree, headings, opt, stats)
       )
     end
     local el = node_name == 'h1' and 'h2' or 'h3'
-    return ('<%s id="%s" class="help-heading">%s</%s>\n'):format(el, tagname, text, el)
+    return ('<%s id="%s" class="help-heading">%s</%s>\n'):format(el, tagname, trimmed, el)
+  elseif node_name == 'heading' then
+    return trimmed
   elseif node_name == 'column_heading' or node_name == 'column_name' then
     if root:has_error() then
       return text
@@ -695,7 +702,7 @@ local function visit_node(root, level, lang_tree, headings, opt, stats)
       return string.format('%s</span>', s)
     end
     return s
-  elseif node_name == 'modeline' then
+  elseif node_name == 'delimiter' or node_name == 'modeline' then
     return ''
   elseif node_name == 'ERROR' then
     if ignore_parse_error(opt.fname, trimmed) then