Summary

support import.defer() for context module:

const file = Math.random() > 0.5 ? "a.js" : "b.js";
import.defer("./dir/" + file);

What kind of change does this PR introduce?

feature

Did you add tests for your changes?

Yes

Does this PR introduce a breaking change?

No

If relevant, what needs to be documented once your changes are merged or what have you already documented?

With #20064 now webpack has full support for import.defer(), we can document that