Expose types for `lib/keys`

[akoreman]

Describe the bug

Currently, we expose lib/keys as an empty type:

ace/types/ace-lib.d.ts

Line 94 in de21d50

declare module "ace-code/src/lib/keys" { }

We should improve this to export the type of this module so that typescript users don't have to define the types themselves for this module.

Ace Version

1.37.1

[akoreman]

Implemented by #5783.