Add UTF16 sources test

[camillobruni]

• Add explicit plan.allowUtf16 setting
• Add unittest that checks that no js/mjs files or preloads accidentally contain two-byte characters
• Fix workloads where with two-byte comments:
  • Use dash instead of em-dash for segmentation / async-tasks URL comment
  • Fix copied bigint-noble license header and use simple double quotes
  • Replace "•" with " dot " in sunspider comments

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[kmiller68]

LGTM, thanks!