ts-spec no longer works in TS 4.8

[geoffreytools]

The fix in #5 caused a regression in TS 4.8.

In this version, Uppercase<string> | Lowercase<string> | Capitalize<string> | Uncapitalize<string> can't be resolved to anything and any conditional type mentioning them makes the problem bubble up, to the point the entire disambiguation algorithm and the two operands of the comparison ultimately fail to resolve as well.