Allow comments in .bazelversion

In the Angular repo, there are some steps you should take to update RBE toolchain whenever you update the bazel version. This would naturally live as a comment in the bazelversion file so you always see it when you update the version.
I suggest ignoring lines that start with `#` (would have to do the same in bazel itself where it understands the .bazelversion file)

Start with the .bazelversion parsing entry point in Bazelisk, then compare it with the corresponding parser in Bazel. The change is complete when lines beginning with # are ignored consistently by both tools and tests cover commented .bazelversion entries.

Written by the indexing model from the issue text.