Add Matt as maintainer of Harmonica

[leouieda]

@mdtanker joined the maintainer team recently and we forgot to go through this checklist from the maintainers guide:

• Add them to the appropriate GitHub team to grant admin access to the repository.
• Add them to the pyproject.toml file so that they show up on PyPI.
• Make sure they have a PyPI and TestPyPI account with 2FA enabled.
• Add them as package owners on PyPI.
• Add them as maintainer in the corresponding conda-forge feedstock repository.
• Have a meeting to talk about their new responsibilities (outlined in
  GOVERNANCE.md).
• When a new package release is required, arrange a video call to walk them through
  the entire release process.

[mdtanker]

Thanks for laying out the steps to be done, I'll open a PR for some of them!