Support PDM: https://github.com/frostming/pdm

I'd like to get Dependabot support for Python projects managed by [PDM](https://github.com/frostming/pdm).

PDM is very similar in usage to Poetry, and also uses `pyproject.toml`, so it could be in conflict with the Poetry file updater 😕
It also has its own `pdm.lock` file.

Just opening for visibility, to see if others are interested 🙂

Start by comparing the existing Poetry file updater with PDM's pyproject.toml and pdm.lock files, as described in the issue. Check how the current updater detects Poetry projects and handles lockfiles; done means Dependabot recognizes PDM projects, avoids conflicting with Poetry support, and updates pdm.lock correctly.

Written by the indexing model from the issue text.