[feat] Add support for [project]-configuration in pyproject.toml

[foxeddev]

Right now, Beet only ready config data from either a beet.json/beet.yml/beet.yaml/beet.toml file or a pyproject.toml file.

1. The pyproject.toml-file is only recognized when it contains a [tool.beet] section, even if the section is empty. Why do you need an empty section? Just read the config always.
2. Beet can optionally read data from a [tool.poetry] section. Most people put their project config in the [project] section though. Why not read that, too?
3. The hierachy is pretty weird. Beet prioritizes pyproject.toml over beet.* files.

[foxeddev]

Btw I would love to try making this myself because I wanna learn stuff, I just read somewhere that you should make an issue before opening a PR.