Convert P2675R1 `<format>` generator from C++ to Python

Followup to #3903 implementing #3446.

This PR is introducing a generator `tools/unicode_properties_parse/format_width_estimate_intervals.cpp`, cited in `<format>`, that processes a Unicode file `EastAsianWidth.txt`. Our other `<format>` generators are all Python, and this one should be too for consistency.