Fonts: fix typing for disabled fallbacks (empty array)
Documentation says:
> To disable fallback fonts completely, configure an empty array:
>
> fallbacks: []
But I get a Typescript error if I do that. This is a simple typing issue, not a behavioral one, so in this PR I am:
- fixing the type so an empty array is allowed, to match the doc
- adding a type test that fails before this change and succeeds after:
> test/types/define-config.ts:150:8 - error TS2322: Type '[]' is not assignable to type '[string, ...string[]]'.
> Source has 0 element(s) but target requires 1.
>
> 150 fallbacks: [],
> ~~~~~~~~~
Partially addresses https://github.com/withastro/astro/issues/13659