@uppy/core: add type tests

[Murderlon]

Ref: #4764

Some tests to make sure types keep working as expected.

[github-actions[bot]]

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[Murderlon]

Can we also add test cases with type errors, to validates that errors would be reported?

Which scenarios do you have in mind? Uppy can throw in some cases but that doesn't have much to do with conditional types.

[aduh95]

Some @ts-expect-error directive over e.g. a call where a mandatory option is missing.

[Murderlon]

Keeping it simple for now. There are many, many options for which we could test endless invalid combinations. And we have Uppy.test.ts already catching a lot of options and how they're used.