Make various typechecking improvements

[AndrewFerr]

Some type-checking improvements that make parts of the code more expressive & easier to work with.

Note that this is based on the config widget PR because it is the source of one of the type annotations added by this PR.

[Half-Shot]

One comment, otherwise LGTM

[AndrewFerr]

Before submitting this, there's a type error added by this change that yarn lint missed which I'll try fixing.

https://github.com/matrix-org/matrix-hookshot/pull/505/files#diff-e9a8de0579bab9c5fea95a16abc38388d4f7b0712ea52c581238f6d4c6fccaa7R132

That change to the type of filters fixes lint errors, but causes mismatched-type errors when calling that function in places like this:
https://github.com/matrix-org/matrix-hookshot/blob/main/web/components/roomConfig/GitlabRepoConfig.tsx#L21

Argument of type 'GitLabTargetFilter' is not assignable to parameter of type 'Record<string, string> | Record<string, never> | undefined'.
  Type 'GitLabTargetFilter' is not assignable to type 'Record<string, never>'.

Basically, I want a type that can cover {}, undefined, and a Record<string, string>, but without using {} as a type, which the linter considers an error.

[AndrewFerr]

Actually, I'll rebase this before merging, since it was based on the tip of the widgets branch.