[CLEANUP] Fix type annotation of ::getSelectors()

[oliverklee]

Also add a native type declaration.

This will need more cleanup and refactoring later on.

[coveralls]

coverage: 56.87%. remained the same
when pulling 854e71a on cleanup/annotations/decleration-block
into fcaea55 on main.

[JakeQZ]

I guess since the array keys can come from the argument passed to setSelectors(), they could be anything.