Make ThemeContext into a proper component

[nyurik]

Please add ThemeProvider to the published code, rather than limiting it to src-doc. Switching themes seems to be a very common scenario, and I suspect its implementation is fairly consistent -- a context consumer that would allow light/dark toggling, and a provider that should use the provided EUI styles with optionally adding a few app-specific ones.

Note that there is a bug -- the NPM package's @elastic/eui/eui.d.ts already includes ThemeProvider and many other src-doc files, thus IDEs suggest to add import { ThemeProvider } from '@elastic/eui'; which later does not work. See #3849

[snide]

This was covered in some internal discussion, but posting here for visibility. Currently theme management is the responsibility of the application, not EUI. This will likely change as we move towards CSS-in-JS solutions, and we'd rather keep the discussion on those issues. Here is a good summary of some recent work around the subject. #3760 (comment)

Gonna close this one, but leave the linked issue (which is a bug) up.