Feat: add css variable support

[The-Code-Monkey]

from what i found before doing

&[data-size="sm"] didnt work but &[data-size='sm'] did so i updated the regex string to support double quotes.

this PR also now adds variable support. Might need a custom css.d.ts file if the as string is not wanted, just due to react not liking inline variables as they are not part of CSSType yet.

fixes #36
fixes #40

[The-Code-Monkey]

@typicode let me know your thoughts and I'll do any tweaks if you want any doing

[typicode]

Thanks for the PR. I feel that things with the current codebase are getting quite complex.

I plan to rewrite the parsing code, I think it can be improved. Also, I will re-organize a few things in preparation of other features.

Since tests are passing, I can merge it and release a new version if you need it?

[The-Code-Monkey]

@typicode yeah I understand, I did need this, but it's up to you whether to merge something like this now or add it when you rebuild things

[The-Code-Monkey]

@typicode any update on this as I have some other things I'd like to suggest but I have built them on top of this pr

[typicode]

I'm making good progress, but I'd like to rewrite docs and iron things out before releasing the next version.

[The-Code-Monkey]

@typicode of course no worries. The only other thing I came across and fixed was I added a list of html element that don't support child element so I added that logic in.

[The-Code-Monkey]

I will wait for your rewrite before trying to add anything else

[The-Code-Monkey]

closing due to further work needed.