fix: support mocked constructors in Vitest v4

[sirlancelot]

See: https://vitest.dev/guide/migration.html#spyon-and-fn-support-constructors

[mcous]

Thanks for the contribution @sirlancelot! Looks like there's a typescript check failing, mind taking a look?

Also, could you please add a test to validate the behavior? Feel free to bump Vitest to v4 in dev dependencies to do so (on the off chance this causes other things to blow up feel free to ping me)

[mcous]

Little update: I messed around with this a little and getting the typing correct is non-trivial! If you haven't already, can you make sure "allow maintainers to push changes" is checked for this PR? (Already set!) I should be able to get this sorted on Sunday

[sirlancelot]

I adjusted the signature and added a test. Hopefully it passes the checks. Thanks for this library!

[sirlancelot]

I didn't want to update Vitest within this project because it would require updating all the unit tests' vi.fn() that use arrow functions. I also wanted to make sure the change is backwards compatible.

[mcous]

Thanks for the fix! This has been released in v0.9.0

I didn't want to update Vitest within this project because it would require updating all the unit tests' vi.fn() that use arrow functions

Thankfully, this was not necessary. It is only necessary to use a function expression in the specific case of using this inside a thenDo. Otherwise arrow functions continue to work as they did previously

I also wanted to make sure the change is backwards compatible.

It took some TypeScript gymnastics (see #34), but even with this change vitest-when continues to support vitest all the way back to 0.x