Add pylint CI and unit tests

[aidnem]

We have no way of validating that the project works at all currently. Someone could completely obliterate functionality and we'd have no idea until we tried to use it next.

To solve this:

• Make the project pass pylint (there are like 20 trivial errors right now)
• Add CI that uses pylint and mypy to check code on PR to main
• Add a basic unit test (To start, just add unit test for filters and paths, we can figure out how to do automated testing for full templates later)
• Make the CI require tests to pass