Split ContextMenu into ContextMenuInner subcomponent

[NyxCode]

See #620
This PR extracts most functionality of ContextMenu to ContextMenuInner, except event handling.
The functionality of ContextMenu should be unaffected by this.

ContextMenuInner can now be used when listening for on:contextmenu on window is undesirable. In the documentation, I added an example for binding the context menu to only a single element.

I'm pretty new to svelte & this library, so please thoroughly review my changes.

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[geoidesic]

Any chance of resolving the conflicts?

[geoidesic]

I tried just using the ContextMenuInner on it's own and I get this error, which I can't interpret:

proxy.js:15 [HMR][Svelte] Unrecoverable error in <ContextMenuOption>: next update will trigger a full reload
ContextMenuOption.svelte? [sm]:79 Uncaught (in promise) TypeError: Cannot read property 'subscribe' of undefined
    at instance (ContextMenuOption.svelte? [sm]:79)
    at init (index.mjs?v=cfb104fc:1660)
    at new ContextMenuOption (ContextMenuOption.svelte? [sm]:129)
    at createProxiedComponent (svelte-hooks.js:245)
    at new ProxyComponent (proxy.js:241)
    at new Proxy<ContextMenuOption> (proxy.js:341)
    at Array.create_default_slot (SceneThumb.svelte:1)
    at create_slot (index.mjs?v=cfb104fc:61)
    at create_fragment (ContextMenuInner.svelte? [sm]:37)
    at init (index.mjs?v=cfb104fc:1675)

[geoidesic]

I would consider calling it ElementContextMenu. As when it is used in isolation without the Global it is not Inner.

I also notice that if it is used without the Global, right-click outside of the context menu does not close it. This is undesirable. I had a go at fixing it but couldn't figure out how @NyxCode