fix: in some cases the syntax file can be evaluated before ftplugin

Two users, @shadowwa and @dmaes reported errors where vim 9.1 was not
happy about the variable g:puppet_display_errors not being defined when
evaluating `syntax/puppet.vim`. They found out that the order the plugin
scripts were loaded did not reflect my current understanding of how vim
loads plugin files: the syntax file was loaded before the ftplugin
script, so the variable was indeed not yet defined.

Since the variable is only used in the syntax script, we can declare its
default value there if the variable doesn't exist (instead of in the
ftplugin file)

I've also changed to using the more common values 0/1 for
binary flags in order to use vimscript's "truthy/falsy" nature of a
variable.

(Closes: #151)

README.md

@@ -12,8 +12,8 @@ Provides
 * Formatting based on the latest Puppetlabs Style Guide
 * Syntax highlighting compatible with puppet 4.x
   * by default, highlights errors: mixing spaces and tabs and bad names. If you
-    don't want this highlighting, add `let g:puppet_display_errors = v:false`
-    (or really any value that's not `v:true`) to your vimrc.
+    don't want this highlighting, add `let g:puppet_display_errors = 0` to your
+    vimrc.
 * Automatic => alignment
   * If you don't like that, add `let g:puppet_align_hashes = 0` to your vimrc.
 * Ctags support

ftplugin/puppet.vim

@@ -22,10 +22,6 @@ setlocal commentstring=#\ %s
 
 setlocal formatexpr=puppet#format#Format()
 
-if !exists('g:puppet_display_errors')
-  let g:puppet_display_errors = v:true
-endif
-
 let b:undo_ftplugin = '
     \ setlocal tabstop< tabstop< softtabstop< shiftwidth< expandtab<
     \| setlocal keywordprg< iskeyword< comments< commentstring<

syntax/puppet.vim

@@ -14,6 +14,10 @@ endif
 let s:cpo_sav = &cpo
 set cpo&vim
 
+if !exists('g:puppet_display_errors')
+  let g:puppet_display_errors = 1
+endif
+
 syn cluster puppetNotTop contains=@puppetExtendedStringSpecial,@puppetRegexpSpecial,puppetTodo
 
 syn match puppetSpaceError display excludenl "\s\+$"
@@ -282,7 +286,7 @@ hi def link puppetFunction        Function
 hi def link puppetDeprecated      Ignore
 hi def link puppetDebug           Debug
 
-if g:puppet_display_errors ==# v:true
+if g:puppet_display_errors
   hi def link puppetInvalidNumber   Error
   hi def link puppetNameBad         Error
   hi def link puppetSpaceError      Error