Add type annotations to runfiles library

[dzbarsky]

PR Checklist

Please check if your PR fulfills the following requirements:

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature (please, look at the "Scope of the project" section in the README.md file)
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Documentation content changes
• [x] Other... Please describe: Type annotations/documentation

What is the current behavior?

No annotations currently

Issue Number: N/A

What is the new behavior?

Annotations

Does this PR introduce a breaking change?

• [ ] Yes
• [ ] No

No, unless we need to support very old python versions without typing in the stdlib?

Other information