Update references from "import assertions" to "import attributes"

[nicolo-ribaudo]

This PR updates all references to import assertions to the new version of the proposal, now called import attributes. The proposal has reached consensus for Stage 3 during the March 2023 TC39 meeting, pending editorial reviews and tc39/proposal-import-attributes#133.

Significant changes:

• import attributes can affect how a module is loaded and can be part of the cache key. This removes the willful violation HTML had in its usage of import assertions, and unblocks CSS and JSON module scripts and CSP #7233
• the keyword has been changed from assert to with. Due to existing support in Chrome/Node.js/Deno the assert keyword will stick around for a while, and the V8 team will investigate the possibility of removing it.

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[GeoffreyBooth]

So as far as JSON imports are concerned, it seems like we're renaming assert to with and that's all that changes? The type is still part of the cache key and required?

[domenic]

Yes, but #7233 will probably change the fetch destination too.