fix(minifier): fix string concatenation for lone surrogates

[h-a-n-a]

Fixes a simple use case in optimizing lone surrogates. Also initialized and pinpointed some places that need a fix in future PRs.

With input:

'\uD83D' + '\uDD25'
`\uD83D` + `\uDD25`

Previously outputs:

"�d838�dd25"
"�d838�dd25"

Now outputs:

"\ud83d\udd25"
"\ud83d\udd25"

Ported some optimization fixes I made in https://github.com/swc-project/swc/pull/10987 and I'm really appreciate the job you all have done. Without the previous investigation made by @overlookmotel, @Boshen and Oxc team, this will not happen.

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[Boshen]

This is a lot of code to support lone surrogates, I'll take a deeper look later.

[codspeed-hq[bot]]

CodSpeed Instrumentation Performance Report

Merging #13229 will not alter performance

_{Comparing h-a-n-a:lone-surrogates-optimization (f1b2f8c) with main (d27a04b)[^unexpected-base]} [^unexpected-base]: No successful run was found on main (e84ae8b) during the generation of this report, so d27a04b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report.

Summary

✅ 34 untouched benchmarks

[h-a-n-a]

This is a lot of code to support lone surrogates, I'll take a deeper look later.

@Boshen Thanks! That would be great and I'd love to fix all the TODOs I listed in the PR, probably in the future PRs. If you need any help, just let me know ;-)

[h-a-n-a]

FYI: we've replaced our implementation here in swc with Wtf8Buf.

See:

1. https://github.com/swc-project/swc/pull/11104
2. https://github.com/swc-project/swc/pull/11144