Added mini.pick backend

[codeinclined]

I've recently started using mini.pick and thought I'd give a shot at adding it as a backend to actions-preview. I mainly just adapted what code was already there for telescope and nui. This is my first time contributing to this repo and writing code for mini.pick, so I hope it's satisfactory. Please let me know if any changes are needed!

Note: mini.pick has built-in caching support, so I didn't attempt to copy the caching stuff that the nui backend has.

I believe this cleans up after itself properly. I didn't thoroughly test that out though. I can if you need me to.

The only issue I've run into is when you do a preview and it hits the code branch where it spits out a hardcoded "preview not available", hitting tab to return to the picker closes the picker. In every other case that I've tested, it works as intended.