Core: Refactor preview rendering out of PreviewWeb

[tmeasday]

Precursor to #17214

What I did

Pulled the rendering part of PreviewWeb out into two classes: StoryRender and DocsRender.

• StoryRender handles the job of "preparing" (ie. import-ing) the story
• Then, depending on whether the mode the story is rendering (which needs to know if the story is docs-only, thus the preparing ahead of time), either renders or converts to a DocsRender
• When modernInlineRender calls renderStoryToElement (one or more times on the docs page), this also creates additional StoryRenders.
• The Preview simply "multiplexes" channel messages that drive re-renders to the StoryRenders and DocRender that are on the screen.

Some notes:

• We don't yet handle the case of "tearing down" the render during preparing (PR to follow).
• We only reload the screen when you tear down a story (change page) not when you reload the story (I think this is intended @ghengeveld?).

In any case this PR is not intended to change functionality, just refactor.

How to test

• Is this testable with Jest or Chromatic screenshots?

See existing tests. I also added some for checking the re-rendering behaviour in modernInlineRender docs mode.

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[shilman]

Can you document the control flow somewhere? It seems like there are a few different layers of indirection now and I'm having trouble following.

[shilman]

does the type DocsContextProps make sense anymore? seems something like DocsContext might be more appropriate (tho that's probably taken)

[tmeasday]

Yeah, I was just mainly moving stuff around but it did occur to me that name was a bit "unexpected".

[tmeasday]

Hmm, it seems to be called that all over the place so I'm inclined to leave it in this refactor