Fix template-no-empty-headings: detect this./@/dot-path component children

GTS component invocations can appear as <this.Foo>, <@foo>, or <ns.Foo>
in addition to PascalCase. The rule's isComponent() check was too narrow,
so children of headings using these forms were not recognized as content
and the heading was falsely flagged as empty.

Author: johanrd

lib/rules/template-no-empty-headings.js

@@ -19,7 +19,15 @@ function isComponent(node) {
     return false;
   }
   const tag = node.tag;
-  return /^[A-Z]/.test(tag) || tag.includes('::');
+  // PascalCase (<MyComponent>), namespaced (<Foo::Bar>), this.-prefixed
+  // (<this.Component>), arg-prefixed (<@component>), or dot-path (<ns.Widget>)
+  return (
+    /^[A-Z]/.test(tag) ||
+    tag.includes('::') ||
+    tag.startsWith('this.') ||
+    tag.startsWith('@') ||
+    tag.includes('.')
+  );
 }
 
 function isTextEmpty(text) {

tests/lib/rules/template-no-empty-headings.js

@@ -38,6 +38,11 @@ ruleTester.run('template-no-empty-headings', rule, {
     '<template><h2><div><span>{{@title}}</span></div></h2></template>',
     '<template><h2><span>Some text{{@title}}</span></h2></template>',
     '<template><h2><span><div></div>{{@title}}</span></h2></template>',
+
+    // Non-PascalCase component forms count as accessible content
+    '<template><h1><this.Heading /></h1></template>',
+    '<template><h2><@heading /></h2></template>',
+    '<template><h3><ns.Heading /></h3></template>',
   ],
   invalid: [
     {