feat(16755): interface stubbing for anonymous implementations

[a-tarasyuk]

Fixes #16755

This PR fixes the second part of the requested changes, the first part was resolved in the #44576

Ideally, this would also be able to stub in the anonymous implementation of a function argument, but that would require inference of the interface being targeted for implementation:

function example(arg: Foo): void { } example({}) example({ field: string, func(): void { throw new Error('not yet implemented') }, }

[sandersn]

Looks good, just a couple of minor refactors to match the other ones already there.