Improve flags types to acknowledge isMultiple and isRequired options

[voxpelli]

This PR fixes two things in the types of flags:

1. Without this PR the type of an isMultiple flag is still reported to be a non-array.
2. All types are right now reported as always being defined, even when both isRequired and default is missing. This PR adds |undefined in those cases + does so in the case where isRequired is a function, as it's kind of hard to figure out what that function will return.

As a separate commit this PR also mirrors the Typescript config from tsd + extends it slightly. It does this for two reasons:

1. With a tsconfig.json in the project tools like VS Code can automatically configure itself correctly and provide correct feedback. Without the tsconfig.json it complained about some of the definition tests.
2. To be able to test |undefined one needs to have strictNullChecks set, else Typescript will just ignore any undefined added in the types.

I also added in noImplicitAny and noUnusedLocals in the tsconfig.json as both of those are helpful and good practice as well.

Somewhat breaking:

The addition of |undefined to all types that are missing both isRequired and default can cause some complaints in the code for those who have strictNullChecks set to true. For everyone else it will cause no change.

These complaints are correct though, but might cause some headaches for eg. those without lock-files, where it can have their CI fail.

[voxpelli]

Fixed the failing tests 🙈 (A downside of being used to husky, one forgets to manually run tests before push)

[sindresorhus]

@NiGhTTraX Would you be able to give this a quick look?

[NiGhTTraX]

LGTM otherwise.