feat!: change default charset to utf8

[chenjiahan]

Summary

Cange default charset from ascii to utf8.

Motivation:

• Smaller output, no need to escape.
• Align with the default value of SWC / Rspack / Vite.
• Align with the default <meta charset="utf-8"> HTML meta tag.

Related Links

• What is reason for output.charset to be `ascii` as default in RSBuild ? #3244
• https://swc.rs/docs/configuration/compilation#jscoutputcharset
• feat!: set esbuild default charset to utf8 vitejs/vite#10753

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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