fix: some remaining cleanups for post processor fixes

[sofisl]

This PR does some final cleanups that I found when fully launching removing the post-processor (with the new combined_gapic_rule), including:

In typescript_gapic_combined_pkg.bzl:

• improves logic for searching for whether a library is ESM by allowing the requested path "esm/src" to be searched for as a glob, rather than one directory-level deep.
• improves logic for templates_excludes. If a library did not pass a template_excludes parameter, then bazel was trying to run a join(" ") with the various listed strings. I added some logic here so that we only do that if it's passed, otherwise not.

In templates:

• Adds a .OwlBot.yaml template. Originally I figured we wouldn't need to migrate this over since we won't be using Owlbot. But I think I jumped the gun a bit, we'll still need to produce it until we can get onto librarian. Eventually we should remove it.
• Re-adds a missing bracket in the CONTRIBUTING file.