expr: replace onig with fancy-regex

[wtcpython]

Replace the onig C-dependency in expr with the pure-Rust fancy-regex crate.
This removes the onig_sys C library dependency while preserving full POSIX BRE,
backreference, multibyte UTF-8, and C locale compatibility.

Closes #1145

[sylvestre]

maybe move this into a new function?

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 153.88 MB (+1.68 MB, +1.10%)

Significant per-binary changes:
  expr       2.76 MB ->    3.77 MB  (+1.01 MB, +36.49%)
  ptx        2.42 MB ->    2.56 MB  (+140 KB, +5.65%)
  csplit     2.38 MB ->    2.51 MB  (+136 KB, +5.59%)
  nl         2.35 MB ->    2.48 MB  (+136 KB, +5.66%)
  tac        2.34 MB ->    2.48 MB  (+136 KB, +5.67%)

[sylvestre]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 153.89 MB (+1.70 MB, +1.12%)

Significant per-binary changes:
  expr       2.76 MB ->    3.79 MB  (+1.03 MB, +37.20%)
  ptx        2.42 MB ->    2.56 MB  (+140 KB, +5.65%)
  csplit     2.38 MB ->    2.51 MB  (+136 KB, +5.59%)
  nl         2.35 MB ->    2.48 MB  (+136 KB, +5.66%)
  tac        2.34 MB ->    2.48 MB  (+136 KB, +5.67%)

https://docs.rs/fancy-regex/latest/fancy_regex/#features

maybe we can disable some features

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)