Catch unresolved partial parameters

Edit the `remark-includes` linter to ensure that all parameters declared
within a partial (using `{{ param }}` syntax) are either (a) assigned by
the user or (b) given a default value. Otherwise, when we move to the
new docs engine, builds will fail on unresolved parameters.

server/fixtures/includes-vars-erroneous-include.mdx

@@ -0,0 +1,5 @@
+## Installation
+
+Here is a test for including variables in an MDX file.
+
+(!install-version.mdx version="10" unsupport="9" !)

server/remark-includes.ts

@@ -244,6 +244,21 @@ const resolveIncludes = ({
         content = content.replace(varRegexp, finalVal);
       }
 
+      // Catch unresolved parameters, which can break docs builds
+      const paramRE = new RegExp(`{{ ?\\w+ ?}}`, "g");
+      const unresolvedParams = Array.from(content.matchAll(paramRE));
+      if (unresolvedParams.length > 0) {
+        const errs = unresolvedParams
+          .map((el) => {
+            return el[0];
+          })
+          .join(",");
+
+        error =
+          `${includePath}: the following partial parameters were not assigned and have no default value: ` +
+          errs;
+      }
+
       return content;
     } else {
       error = `Wrong import path ${includePath} in file ${filePath}.`;

uvu-tests/remark-includes.test.ts

@@ -483,6 +483,27 @@ Suite("Resolves template variables in includes", () => {
   assert.equal(result, expected);
 });
 
+Suite("Throws an error if a variable is unresolved and has no default", () => {
+  const value = readFileSync(
+    resolve("server/fixtures/includes-vars-erroneous-include.mdx"),
+    "utf-8"
+  );
+
+  const out = transformer(
+    {
+      value,
+      path: "/content/4.0/docs/pages/filename.mdx",
+    },
+    { lint: true }
+  );
+
+  assert.equal(out.messages.length, 1);
+  assert.equal(
+    out.messages[0].reason,
+    "The following partial parameters were not assigned and have no default value: {{ unsupported }}"
+  );
+});
+
 Suite(
   "Resolves relative links in partials based on the path of the partial",
   () => {