Rename `google-java-format-diff.py` to `google_java_format_diff.py` for **Module Import** Compatibility

[sugamadhiakri]

This pull request renames the file google-java-format-diff.py to google_java_format_diff.py and updates all occurrences in the codebase and documentation to reflect this change. The primary motivation for this change is to enable the script to be imported as a Python module, which is not possible with hyphens in the filename.

Changes Made:

1. Renamed google-java-format-diff.py to google_java_format_diff.py.
2. Updated all occurrences of google-java-format-diff.py in the code, README, and LICENSE to google_java_format_diff.py.

Benefits:

1. Enables importing google_java_format_diff.py as a module, facilitating its use in other Python scripts and improving modularity.
2. Enhances the script's compatibility with Python standards for module names.

Please review the changes and let me know if any further modifications are needed. Thank you!

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[sugamadhiakri]

resolves #1120