fix: support typing overloaded functions

[mcous]

Overview

TypeScript's built in Parameters and ReturnType utilities do not understand overloaded functions. This makes stubbing overloaded functions quite difficult with vitest-when, because calledWith, thenReturn, et. al. will get typed to accept the last overload rather than a union of all overloads.

This PR adds improves the typing for overloaded functions in two ways:

• calledWith will accept a union of all overloaded arguments
• thenReturn/thenResolve will accept the specific return type given the arguments used in calledWith

Change log

• Add typing support for overloaded functions
• Add typing tests to make sure we support vanilla vi.fn(), untyped functions, non-overloaded functions, and overloaded functions
• Improve the README