[Nicolò Ribaudo (nicolo-ribaudo)]

This proposal is currently at Stage 3.

This PR only needs to add parsing support for the proposal:

• it does not support downleveling it, as it's not possible to transpile it to older ESM versions
  • it would be possible add support for transpiling to CommonJS, if needed.
• it does not introduce any type-checking changes, as the object created by import defer * as ns is meant to "look like" the one created by import * as ns

As you'll see from the code, whether a module is deferred or not is not a boolean but an enum. That's because of the import source proposal: there is no PR for it yet, but once it will be implemented it should be done by adding one more possible "import phase" to this enum. A question I have is if that should be an enum, or just an union of undefined | DeferKeyword (which will one day become undefined | DeferKeyword | SourceKeyword).

Fixes #59391

This patch was originally written by Ujjwal Sharma (@ryzokuken)