Allow component names like <Foo.Bar.Baz>?

[Rich-Harris]

Was just playing around with an interpretation of React's createContext/useContext API:

https://svelte.dev/repl?version=3.0.0&gist=2a110c18df550964f9b31aa2425eaaf1

I think it works pretty well, if you prefer that sort of thing to the standard context API, but it might be nice if App.svelte looked like this instead:

<script>
	import ChildComponent from './ChildComponent.svelte';
-	import { Provider } from './ThemeContext.js';
+	import ThemeContext from './ThemeContext.js';
	
	let theme = 'light';
</script>

<select bind:value={theme}>
	<option>light</option>
	<option>dark</option>
</select>

<ThemeContext.Provider value={theme}>
  <ChildComponent />
</ThemeContext.Provider>

I can see this being useful in other scenarios as well, e.g. if a package exported multiple components that were designed to be used together:

<script>
	import * as Funky from 'funky-ui';
</script>

<Funky.Dropdown>
	<Funky.Option>one</Funky.Option>
	<Funky.Option>two</Funky.Option>
	<Funky.Option>three</Funky.Option>
</Funky.Dropdown>

[Conduitry]

This was implemented in #2743.