This CL introduces an 'import' statement. It does not produce any AST
node. The AST contextual directly collects what source id imports what other
source id.

Currently the import map is unused. In the future, import syntax will be
used to implement partial compilation.

Bug: v8:7793
Change-Id: I5f09e6254d7ca2e7bc1a93d2e2d82e202cafc8ef
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1649357
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62080}