pre-commit: Recognise piskel files via exclude_types

[wjt]

My upstream pull request was merged:

• Add .piskel extension pre-commit/identify#541

The maintainer chose to edit my contribution to not add the image tag
to piskel files:

the image tag was an idea that didn't really pan out so I'm just
going to remove it here

Of course that's their prerogative as maintainer!

Unfortunately this means that my plan from commit
e92fd6d, to exclude all image files
with exclude_types: [image], is not going to work. Instead we have to
add the piskel type explicitly to the list, and also any further
text-based image file formats that we find in future.

Add piskel to the exclude list. Replace the image type with svg:
svg is the only format with that type, and since that tag will not be
applied to any new formats, it's clearer to be explicit.

Remove the regex branches that match piskel files by name.

If you have pre-commit set up locally, this change will cause the hook
to fail:

=====> Type tag 'piskel' is not recognized.  Try upgrading identify and pre-commit?

If you installed pre-commit with uv tool install you can run:

uv tool upgrade pre-commit

[github-actions]

Play this branch at https://endlessm.github.io/threadbare/branches/endlessm/wjt/pre-commit-recognise-piskel-files-via-exclude-types.

(This launches the game from the start, not directly at the change(s) in this pull request.)

[manuq]

Thanks for the instructions for updating the pre-commit hook.