fix: typings of ctx.skip() as never

[sirlancelot]

Since skip throws an error, its return type should be set to never so that TypeScript & linters can determine that this code path ends.

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[AriPerkkio]

How does this change come up in practice when writing test cases?

[sirlancelot]

When I was writing a test that used skip() I didn't realize that it would terminate that code path, so I instead wrote return skip() to indicate that the code path is done. With a type signature of never I (as well TypeScript) would've known this already.