fix: perf bug for large replace source

[SyMind]

Fix web-infra-dev/rspack#9137

In Rspack's source code, the check_content_at_position function is used to check whether the sourceContent in the source map matches the original source code. If they match, additional mapping information can be added.

The check_content_at_position function uses column (character index) to extract substrings from sourceContent. Since column is a character index and strings are stored as byte indices, a conversion from byte index to character index is required. This conversion process has become a performance bottleneck.

By caching the conversion results from byte index to character index, redundant calculations are avoided, thereby improving performance.

[codspeed-hq]

CodSpeed Performance Report

Merging #156 will not alter performance

_{Comparing SyMind:fix-perf-bug (4649d2a) with main (09aa874)}

Summary

✅ 6 untouched benchmarks
🆕 1 new benchmarks
⁉️ 1 dropped benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	complex_replace_source	N/A	60 ms	N/A
⁉️	replace_large_minified_source	118.6 ms	N/A	N/A