Improve Button type definitions

[fallaciousreasoning]

This does the following:

1. Makes it possible to passing through button attributes.
2. Updates the type definitions such that the button is either a button or a link.
3. Adds the type definitions for the Button/A tag as optional props.

It does not:

1. Add support for events.

@petemill what do you think about this as an approach?

[fallaciousreasoning]

Hey @AlanBreck do we want to try and land this?

[AlanBreck]

Sounds good to me!!

[fallaciousreasoning]

All right, I've fixed the merge conflict and the formatting. Anything else need doing?

[AlanBreck]

Not that I can think of. The only thing that would be nice in the future is if there's any opportunity to simply for future component authors. No idea how, though. 😂

[fallaciousreasoning]

Yeah, It's definitely pretty awkward at the moment - hopefully Svelte improves this :D

[fallaciousreasoning]

@petemill what are your thoughts on this?