Typing the theme function argument

[kaisermann]

The current type for the theme exported function was defined in #169 as

export type ThemeFn = <T = string>(arg?: string | TemplateStringsArray) => T

Can a proper type be added to this function so we can get some autocomplete suggestions and type-checking for typo prevention?

I know we can override the type with a declare module 'twin.macro' in a .d.ts file, but I'm not sure if we could have the parsed config available as a type.

I also considered the Tailwind IntelliSense VSCode extension but couldn't find anything specific. It does seem to check for theme written inside .css files by default, though.