new venv syntax

[jdx]

I think I want to move venv configuration for python/poetry into the [env] section rather than putting it on individual tools:

[env]
_.python.venv = "~/.venv/myvenv"
_.poetry.pyproject = "pyproject.toml"

this would make it still driven by the plugin (allowing other plugins to do similar things), but also make it possible to use with the system python resolving #1248.

More importantly though, this just feels like the more correct place to put it since it's configuring the "environment"—not the python installation. That said it will depend on that so a bit of care around making sure if there is a python version set it needs to be installed before setting the venv.

[rsyring]

I don't follow mise development close enough to know the genesis of the _ environment variables. That being said, I find it odd the settings would be there and not a part of the tool.

[camsteffen]

I think it's good that [env] has the single purpose of setting environment variables. It's analogous to a .env file.