Types do not work with latest TypeScript version

[ehmicky]

It appears that the latest TypeScript version make the current types fail with:

TS2590: Expression produces a union type that is too complex to represent.

This was not detected by tsd because we use an older version which uses TypeScript 5.2.2. When upgrading tsd, TypeScript 5.4.5 is used instead, and the above error shows up.

I have done some heavy (and hours-long) debugging and it appears to be a TypeScript bug, which I filed in microsoft/TypeScript#58268.

Since this might take a while to fix in TypeScript, and we want to support former versions of TypeScript, the next step is to find a workaround so the above error does not show up.