feat(bigframes): Transpiler supports more string ops

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request adds support for compiling f-strings, nullity checks (such as 'is None' and 'is not None'), and various string operations (e.g., capitalize, islower, isupper) in Python UDFs. Feedback is provided regarding the Polars compilation of 'IsLowerOp' and 'IsUpperOp', where the current regex patterns incorrectly require the entire string to consist only of letters, failing to match Python's native behavior which allows non-cased characters.