Clarify worker problem statement, rename to 'Source Text Phase Imports'

[guybedford]

As discussed in the previous meeting, this restructures the proposal to focus first and foremost on the worker problem statement exactly as it has been doing, but with a bunch of simplifications around presenting that and putting the statement first.

The background mentions how in our overall layering work this is seen as a good next step due to the importance of the worker design constraints.

I've kept the proposal layering section, with rewriting for applying to both source and instance.

All of the use cases touched on in the use cases section should be covered by the motivation now, but please let me know if there is anything else there we should keep - I entirely removed the capability section as I wasn't sure how to frame this into the worker problem statement, but I'm open to bringing it back in some form if necessary.

The renaming to 'Source Text Phase Imports' is to indicate that the exact phase definition is not yet fully clear for the Stage 1 proposal phase, to give us some design flexibility here.

[nicolo-ribaudo]

Looks good 👍

[nicolo-ribaudo]

Suggested change

	# Source Text Phase Imports
	# JavaScript Phase X Imports

it really sounds as if "Source Text" is a new phase that we never mentioned before, while what we are actually doing is just not picking a phase yet. Maybe this makes it more clear?

[guybedford]

Thanks, sure I've changed the proposal name to ECMAScript Module Phase Imports here.