fix(vue-imask): Add initial typescript definition file for vue-imask

[AustinGrey]

Currently only useIMask has real type safety, all other exports are given explicit 'any' until their types can be determined.

I'm unsure what if any contribution guidelines there are - I really just want to be able to use the useIMask composition function without a shim, otherwise compilation fails.

[AustinGrey]

This may close (or at least partially works towards) #286