Adding pre-commit (for linting) to test dependencies.

[alxmrs]

Fixes #668.

[tomwhite]

I'm not sure we need to add pre-commit for testing do we? Perhaps we could use https://pre-commit.ci/, or maybe we could add another GH action (or extend/modify the mypy one). Might be worth seeing what other projects do.

[alxmrs]

Hmm. Maybe a better fit would be to add an extra just for linting? Then, when we eventually add this to CI, the lint script can run in it's own check? WDYT?

[tomwhite]

If possible, I'd copy what another project is doing rather than coming up with a new way of doing things.

A quick look shows:

• Bio2zarr has a CI job to run pre-commit
• Xarray uses precommit.ci (see e.g. here), as does VirtualiZarr

[tomwhite]

Thanks @alxmrs!