feat(build): chunk importmap

[sapphi-red]

This PR implements build.chunkImportMap option, which utilizes import maps feature to optimize chunk caching efficiency.
This PR is based on #16552 and uses Rolldown's experimental importmaps feature. Huge thanks to @bhbs who have researched this topic and gave us the insights.

The implementation in the Vite core relies on import.meta.resolve. For that reason, only browsers with import.meta.resolve support are supported when this option is used.
To align with this, plugin-legacy now considers import.meta.resolve to be "widely-available". In other words, browsers without import.meta.resolve will use the legacy chunks instead of the modern chunks. The plugin-legacy support for this feature is implemented by using the import map support in SystemJS.

Note that experimental.renderBuiltUrl currently does not work with this option. To properly support the combination of that option + this option, it requires more effort to design how to handle absolute URLs (e.g. how to support the case where renderBuiltUrl returns an absolute URL that should be mapped by the import map).

Also note that this optimization currently does not apply to CSS and assets. But this shouldn't affect largely as it will only invalidate the JS file that is importing them and would not affect the JS files that transitively imports the changed CSS / asset.

fixes #6773
fixes #10636
close #16552

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commit: aed7c01