perf(scripts): improve sound export throughput

[cmdr-chara]

Summary

• Improves ExportAllSounds.csx throughput by caching audio group lookups, avoiding repeated directory creation, and writing/copying export jobs with bounded parallelism.
• Preserves output filenames and contents for the tested Undertale export case.

Testing

• Compared upstream script vs this change on UNDERTALE/data.win.
• Output matched: 443 files, 128,007,271 bytes, 0 missing/extra/changed files by SHA-256.
• 3-run median: 4412 ms -> 4082 ms (~7.5% faster).

Risks

• Tested with Undertale data.win; other GameMaker titles may exercise different external audio layouts.

[cmdr-chara]

@colinator27 Could you please check the code? This script was improved in my WinUI fork, but I'd like for it to be included here as well.