Add a languages-contrib package

[orta]

Adds a new package, which isn't included by default, which accepts pretty much any language

[octref]

What is that for?

[orta]

The same infra, but for anything outside of #55

[octref]

Do we need it? I think even with loading all grammars it's not that slow. Maybe for #22 we need it though, but we can also introduce an API which loads grammars on demand.

Now I kinda like these being auto generated weekly:
https://github.com/shikijs/shiki/tree/master/packages/languages/data
https://github.com/shikijs/shiki/blob/master/packages/languages/src/lang.ts

[octref]

Defer until we need it.