fix: fixes UTF-16 encoding issues

[SyMind]

This PR fixes UTF-16 encoding issues in source map generation by correctly handling multi-byte Unicode characters. The changes ensure that column positions in source maps are calculated using UTF-16 code units instead of UTF-8 bytes, which is required for proper JavaScript source map compatibility.

• Replaces all uses of Cow<'a, str> with &'a str in chunk processing to simplify the API
• Updates column offset calculations to use UTF-16 code unit counts instead of byte offsets
• Refactors remaining replacement handling to process replacements individually instead of concatenating them

[codspeed-hq]

CodSpeed Performance Report

Merging #202 will not alter performance

_{Comparing perf-on-chunk-to-str (5dded2e) with main (446ceaf)}

Summary

✅ 11 untouched
⏩ 6 skipped¹

Footnotes

1. 6 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. ↩

[Copilot]

Pull Request Overview

This PR fixes UTF-16 encoding issues in source map generation by correctly handling multi-byte Unicode characters. The changes ensure that column positions in source maps are calculated using UTF-16 code units instead of UTF-8 bytes, which is required for proper JavaScript source map compatibility.

• Replaces all uses of Cow<'a, str> with &'a str in chunk processing to simplify the API
• Updates column offset calculations to use UTF-16 code unit counts instead of byte offsets
• Refactors remaining replacement handling to process replacements individually instead of concatenating them

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/replace_source.rs	Core fix for UTF-16 column calculations; refactored replacement handling logic; added comprehensive test coverage for multi-byte Unicode
src/original_source.rs	Updated column calculations to use UTF-16 encoding; added test for multi-unit UTF-16 characters
src/helpers.rs	Changed OnChunk type signature from Cow<'b, str> to &'b str; updated all call sites
src/cached_source.rs	Minor reordering of struct fields

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