test(lsp): add hemplate highlighting fixture

Add repo-owned hemplate highlighting fixture and golden capture contract without editor packaging or a second parser.

req: diagnostics/004

req: diagnostics/008

docs/fixtures/hemplate-highlighting/hemplate.heml

@@ -0,0 +1,18 @@
+<main data-hemx-root="demo">
+  <form data-hemx-form="search" data-hemx-handle="run_search">
+    <input +class="self.search_class" name="query" />
+  </form>
+
+  <section h-if="self.show_results">
+    <template h-for="result in &self.results" h-key="result.id">
+      <article data-hemx-slot="result_row">
+        <h2>{+ result.title +}</h2>
+        <div>{+= result.summary_html =+}</div>
+      </article>
+    </template>
+  </section>
+
+  <template h-match="self.state">
+    <p h-case="ViewState::Empty">No results</p>
+  </template>
+</main>