Implement Context.withImports

[Simn]

See #10537

Still have to add API dorkumentation.

@back2dos I've changed the API to use Array<String> for both because that's easier to use. Is there any reason it shouldn't be like that?

[back2dos]

I guess I based my suggestion on get:LocalImports() and getLocalUsing(), but turning the returned data into strings is easy enough, if that's where the user gets the data from. Otherwise just chucking in strings is definitely more convenient ;)

[back2dos]

Oh, I was also curious to know if * imports (or imports of modules with statics) will shadow vars (and current fields with implicit this / TheClass) or not? Not that I lean in any particular direction, but I'd say it's the kind of thing that should be specified one way or the other ;)

[Simn]

Resolution order will behave as if these were normal imports.