🧪 Tests: Add tests for remaining functions in src/flags.ts

Bug Report Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

#6 added a first round of unit tests. Let's add some more tests! 🙌

The src/flags.ts file is not yet tested. This issue tracks adding in tests for its exported functions.

Additional Info

See #6 for examples of tests: including those that work with AST nodes and/or type checking.

There might be existing tests in https://github.com/ajafff/tsutils that are relevant here. If you do find them, it's fine to copy & paste them over - as long as there's a comment like this above any area of copy & pasted code (or on top of a file if the entire file is copy & pasted):

// Code largely based on https://github.com/ajafff/tsutils
// Original license: https://github.com/ajafff/tsutils/blob/26b195358ec36d59f00333115aa3ffd9611ca78b/LICENSE

Start with the exported functions in src/flags.ts, then read the tests added in #6 for patterns involving AST nodes and type checking. Review relevant tests in tsutils if useful, preserving the requested attribution for copied code. Done means the remaining exported functions have focused unit-test coverage for their relevant cases.