feat: Use custom pre-commit config file

[tdurk93]

secureli-373

Migrates .pre-commit-config.yaml into the .secureli/ directory. This directory was previously ignored by git, so I moved its existing contents into .secureli/local/ and told git to only ignore that subfolder. Using a nonstandard path for .pre-commit-config.yaml will allow our tool to not destroy any existing configuration/hooks, and allows us to sidestep much of the complexity of attempting to merge our hooks with existing hooks.

There are a lot of repeated hardcoded strings in this PR. I'm inclined to not do too much refactoring/rework to clean that up, since I'd like to do some significant refactoring in the near future.

Open Questions

• I opted to keep the file name the same, but I don't love having a hidden file in the hidden folder (feels excessively "hidden"). I'm open to suggestions of whether pre-commit-config.yaml is better than .pre-commit-config.yaml.
• This will break existing installations of secureli, so we should probably think about adding an automatic migration path prior to merging this. When secureli is invoked in a repo with an existing secureli installation, the user is met with this prompt: seCureLI has not yet been installed, install now? [Y/n]:

Testing

• Existing unit tests pass
• Manually tested by running secureli init in a new repository, then validated the contents & paths of the generated files.

[tdurk93]

This PR still needs some unit tests but otherwise I think it's pretty much ready for review.