grep: warn on leading ERE repeat operators

[wondr-wclabs]

Closes #23.

GNU grep accepts leading ERE repeat operators as a warning case rather than a hard regex compile error. The operator is effectively ignored, so a pattern like ? or {2} becomes an empty match and still selects the input line.

This keeps the compatibility change deliberately narrow: instead of disabling Oniguruma’s invalid-repeat handling globally, it detects only the leading ERE repeat forms described in the issue, emits the GNU-style warning, and compiles the remaining pattern normally. That preserves existing BRE behavior such as treating *foo as a literal basic-regexp pattern, and it avoids changing unrelated independent-repeat cases in the regex engine.

The regression test covers ?, *, +, {2}, and {,2} in ERE mode, plus a BRE *foo control case.

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

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_{Comparing wondr-wclabs:codex/ere-leading-repeat-warnings (0f16075) with main (e925ff4)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩