Review typing of CSS values

[peduarte]

For example, gap can't be a number.

It may be worth referring to this https://github.com/system-ui/theme-ui/blob/f5dd5b3ee601909a52ceb14cbd5fdd1857364ef1/packages/css/src/types.ts#L14-L22

[peduarte]

At the moment, numbers are not supported in most properties. This is because initially we didn't want to allow people to set a unitless number, and have it being converted to px value.

It seems that we've changed our minds about this, @colmtuite can you confirm, that now we do want to support unitless numbers? (with the exception of line-height)

@hadihallak maybe its worth doing this together when you implement the functionality?

[peduarte]

Since we dont rely on css-types our implementation would probably be different

I've got it to work by changing the types to this:

export type GapProperty<TLength> =
  | Globals
  | TLength
  | "normal"
  | ((string | number) & {});

But will hold off until we can decide what to do

[peduarte]

Done in #206