ci: move public-api goldens to goldens directory (#35768)

Moves the public api .d.ts files from tools/public_api_guard to
goldens/public-api.

Additionally, provides a README in the goldens directory and a script
assist in testing the current state of the repo against the goldens as
well as a command for accepting all changes to the goldens in a single
command.

PR Close #35768

tools/ts-api-guardian/lib/main.ts

@@ -57,7 +57,7 @@ function ensureDirectory(dir: string) {
  */
 function isDirectory(dirPath: string) {
   try {
-    fs.lstatSync(dirPath).isDirectory();
+    return fs.lstatSync(dirPath).isDirectory();
   } catch {
     return false;
   }