Thanks for the PR, looks like a great starting point!
Let me add a thought, this seems to me more like a setAll than a proper reset, which I would expect also setting the new default values.

Doing this (in two separate moments):

reset({ firstName: "foo" }) // Passing an object - set the new default
reset() // No parameters - reset to default

Should end up with the form this the state { firstName: "foo" }.
At least as a default behaviour, then nothing stops us from adding an extra parameter with a flags, similar to RHF, to control that.

What do you think?