Intent to prototype: Deferred Imports Evaluation

[Yoshi Cheng-Hao Huang]

Summary:

`import defer` allows lazily evaluating ES modules, to reduce the performance impact of modules that are not actually needed from the startup.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1898544

Specification: https://tc39.es/proposal-defer-import-eval/

Standards Body: ECMA

Platform coverage: ALL

Preference: javascript.options.experimental.defer_import_eval

DevTools bug: n/a

Link to standards-positions discussion: n/a

Other browsers:

Chrome: https://issues.chromium.org/issues/398218423

Webkit: https://bugs.webkit.org/show_bug.cgi?id=282882

web-platform-tests: n/a, it is tested by test262

test262/language/import/import-defer

[Nicolas Chevobbe]

Thanks Yoshi for working on this

Note that we do have a DevTools bug (https://bugzilla.mozilla.org/show_bug.cgi?id=1898549), to check that scripts using deferred imports can still be used properly in the Debugger