Allow CLI output to a new path

[dajiaohuang]

The output option is documented for writing decompiled results to a file or directory, and the implementation creates missing destinations. The CLI currently rejects any new destination during argument parsing because the option requires the path to exist.

Allowing missing output paths restores the documented behavior while retaining file and directory validation for existing paths. Added a regression test covering a new output file path.

Validation:

• python -m compileall -q uncompyle6 pytest/test_cli.py
• click path conversion accepts a new output path
• git diff --check

[rocky]

@dajiaohuang I had hoped to be able to look at this and the other PR this week.

Unfortunately, my schedule right now is that I can't look at it until sometime next week.