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

goldens/BUILD.bazel

@@ -0,0 +1,8 @@
+package(default_visibility = ["//visibility:public"])
+
+filegroup(
+    name = "public-api",
+    srcs = glob([
+        "public-api/**/*.d.ts",
+    ]),
+)