fix(highlights): match constants with digits and leading underscores

[dwo]

The constant highlight regex ^[A-Z][A-Z_]*$ did not allow digits or leading underscores, so identifiers like HTTP2, SHA256, _PRIVATE and __MAX_RETRIES were not highlighted as constants despite following PEP 8 naming conventions.

Updated the regex to ^_*[A-Z][A-Z_0-9]*$ and added highlight tests for identifier naming conventions.