Add push, join_*, and empty methods to ParsedPath. (#23650)

# Objective

Fixed #13227

## Solution

This implements the 'mutates self' variant of the API, aside from
`join`. This is meant to mirror `PathBuf::push` vs `Path::join` - one
mutates, the other returns a new value.

## Testing

- Added doctests that verify basic behavior
- May want either examples or unit tests on the various push methods
- Tested on Windows, but that shouldn't matter

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Co-authored-by: Carter Anderson <mcanders1@gmail.com>
Co-authored-by: François Mockers <francois.mockers@vleue.com>