feat: implement defineMutation

[posva]

Currently, it doesn't work like defineQuery(). It probably requires the mutation store first to be ran in a detached effectScope

[Barbapapazes]

Isn't this already done with https://pinia-colada.esm.dev/guide/mutations.html#Reusable-mutations?

[posva]

I think I have to go over it again. It might just work fine now, I remember I reworked how mutations were saved in the cache