[Stdlib] Add os.path.normpath

[BlueDestination]

Linked issue

Fixes #2962

Type of change

• New feature or public API (requires prior proposal or issue approval)

Motivation

normpath is a standard POSIX path utility needed by other stdlib modules, such as tempfile (which has a TODO referencing it).

What changed

Added os.path.normpath and a test file.

Testing

Added test_normpath.mojo and all tests pass.

Checklist

• The linked issue above has been reviewed by a maintainer and is
  agreed-upon, or this is a trivial fix that does not need prior
  approval
• PR is small and focused — I've split larger changes into a sequence of
  smaller PRs where possible (see
  pull request sizes)
• I ran ./bazelw run format to format my changes
• I added or updated tests to cover my changes
• If AI tools assisted with this contribution, I have included an
  Assisted-by: trailer in my commit message or this PR description (see
  [AI Tool Use Policy]

Assisted-by: AI