TypeScript: Improve typings of interpolations

[Igorbek]

This PR slightly improves TypeScript support (typings) of interpolations.

Rationale

There're two very different contexts where styles are being created: global (injectGlobal, keyframes functions) and component (styled function). In the global contexts, functions as interpolations are not accepted. Indeed, there's nothing to use - no props available.
So the typings of these functions already make this distinctions, injectGlobal and keyframes do not accept functions as interpolations.
The issue comes up when css needs to be used. Since it is meant to be composable and can be used inside both contexts, we need to know what types of interpolations are hidden inside.

Changes

• an overload of css has been introduced which only takes simple interpolations
• introduced 'flatten' interpolation type to use as a result of css (which flattens passed array interpolations)
• also made possible to pass arrays as interpolations which wasn't working before
  • Note, that due to TypeScript's circular-reference restrictions, deeper nesting of arrays isn't allowed

Now using function interpolations that came from css is also restricted:

Upcoming

I'm going to make another PR with better theme support in TypeScript. So you could bundle all the changes in the next release.

[mxstbr]

Wow this is awesome. I'll leave it up to @patrick91 to review the code, but this sounds like a useful change to me!

[patrick91]

LGTM! :)

[mxstbr]

Let's ship this when the second PR is in?

[Igorbek]

@mxstbr yeah, let's bundle them together.