Autodoc: only group structs under "namespaces"

The old heuristic of checking only for the number of fields has the
downside of classifying all opaque types, such as `std.c.FILE`, as
"namespaces" rather than "types".

lib/docs/main.js

@@ -427,15 +427,12 @@
           const member_category = wasm_exports.categorize_decl(member, 0);
           switch (member_category) {
             case CAT_namespace:
-              if (wasm_exports.decl_field_count(member) > 0) {
+              if (!wasm_exports.decl_is_struct(member) || wasm_exports.decl_field_count(member) > 0) {
                 typesList.push({original: original, member: member});
               } else {
                 namespacesList.push({original: original, member: member});
               }
               continue member_loop;
-            case CAT_namespace:
-              namespacesList.push({original: original, member: member});
-              continue member_loop;
             case CAT_global_variable:
               varsList.push(member);
               continue member_loop;

lib/docs/wasm/main.zig

@@ -578,6 +578,21 @@ export fn decl_file_path(decl_index: Decl.Index) String {
     return String.init(string_result.items);
 }
 
+export fn decl_is_struct(decl_index: Decl.Index) bool {
+    const decl = decl_index.get();
+    const ast = decl.file.get_ast();
+    switch (decl.categorize()) {
+        .namespace => |node| {
+            const node_tags = ast.nodes.items(.tag);
+            if (node_tags[node] == .root) return true;
+            const main_tokens = ast.nodes.items(.main_token);
+            const token_tags = ast.tokens.items(.tag);
+            return token_tags[main_tokens[node]] == .keyword_struct;
+        },
+        else => return false,
+    }
+}
+
 export fn decl_category_name(decl_index: Decl.Index) String {
     const decl = decl_index.get();
     const ast = decl.file.get_ast();