Multiple import maps

Import maps currently have to load before any ES module and there can
only be a single import map per document. That makes them fragile and
potentially slow to use in real-life scenarios: Any module that loads
before them breaks the entire app, and in apps with many modules the
become a large blocking resource, as the entire map for all possible
modules needs to load first.

This implements https://github.com/whatwg/html/pull/10528 to solve that.

Change-Id: I54e1b9cdfe989d61c85d73a5fd384f860273ad9a
Bug: 358379381
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5776262
Commit-Queue: Yoav Weiss (@Shopify) <yoavweiss@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378943}