Start on party sheet v2

[CarlosFdez]

This is in development and does not have the final refactor pass. It is here as a show and tell of context and tab management. You can see a use of it in overview-tab.svelte.

V12 doesn't have overridable themes, so this has to be viewed with the light theme or it'll be even more horrendous.

I am currently playing with the current ideas in my head. If there is approval I'll do them, but at this point I'm trying to find out if they're bad ideas:

1. Implementing a _prepareState() method that _prepareContext() calls. This is a similar paradigm to handlebars part contexts, kinda.
2. Having BaseSvelteState instead be a separate $appState store that is separate.

I also need to fix window repositioning on re-render. We need to find a way to await until svelte is done re-rendering in _replaceHTML(). It may be doable with the tick() function.

The goal is to avoid deep nesting of "global" props such as user, foundry app, tab groups, and editable. It should not ever be used for regular props that are specific to a sheet, which is why I am considering the split.