Improve IStyle type about fela

[hedwiggggg]

Experimented a bit.

This could be a implementation for the new IStyle:
Playground Link

And this for the new IStyle, using fela-plugin-fallback-value:
Playground Link

[hedwiggggg]

And as you can see here: Playground Link

This change shouldn't break anything, because the legacy nested prop would still be possible.

[hedwiggggg]

Thought about it a little bit more, there is a downside, called declaration merging, see here (typescript docs).

The problem with that is, that plugins can't extend the IStyle type anymore.
Didn't happened yet, but would be nice for the legacy fela-plugin-typescript types and neccessary when using fela-plugin-fallback-value.

Came up with a workaround, seems to be working fine: Playground Link

[hedwiggggg]

Using these changes in my project I've discovered a weird behavior using the declaration merging for the plugins.
Guess it's because an empty interface is equivalent to {}, which is equivalent to "any non-nullish value".

That lead to the following behaviour:

const _test: IStyle = []; // ok, but should throw an error

This can be fixed by providing an optional 'brand' property, so the empty Extension interface won't be equal to {}.

See Playground Link

Edit: Updating the PR now.

[hedwiggggg]

Found another small bug, demonstrated here:

Playground Link

Edit:

And another related bug: Playground Link

Got this as a workaround: Playground Link

Im just exposing IStylePrimitiveExtension as a mergable type. So plugins could hook into that, to extend the primitive type.
For example the following case: Playground Link