Anatomy schema improvements

[lukewarlow]

Fixes #845

Legacy Anatomies:

New Antomies:

[lukewarlow]

Blocked by #850

[lukewarlow]

Follow up issue to work out the UI improvements #874

[lukewarlow]

See #885 for usage of this new schema to see if there's any core requirements missing. For me it's the "slot" concept that I think does need to exist. It largely doesn't exist in browsers atm (aside from default slot) but it does get uses for <selectlist> and for <details>. I also imagine it's a much bigger part in other design systems.

My gut feeling is that the current approach doesn't demonstrate this level of flexibility in the anatomies.

[lukewarlow]

@gregwhitworth I've been looking into using that custom element manifest for our definitions and I think it's definitely viable but it would be more a thing of importing defintions of specific things and incorporating our own spin on it. (The manifest can't be used for nesting of structure as a wholesale thing for example).

Gonna require me to sit down and give it some thought how best to incorporate it all so for I think for now we should try and get this and the browser anatomies merged so we've got something we can use for now. Then I can make a follow up that adjusts it to utilitise the custom element manifest definitions where possible.

[lukewarlow]

@gregwhitworth are you able to approve this when you've got the time. Then we can land the basic browser data as a follow up.

I will separately take a look into a slightly bigger refactor using the custom element schema but that will require a bit more work and I don't want to block having the browser data available.