Refine notes on worker interactions, include section on external import maps

[guybedford]

This updates the worker importMap strawman to include:

• A note on supporting URL importmaps
• Noting service worker and dedicated worker interactions, and that the field may be excluded for them initially
• Also creating a new section for external import maps, to flag this implementation effort and in this context

[guybedford]

/cc @nicolo-ribaudo