Create a rust_python_ast crate

[charliermarsh]

This PR productionizes @MichaReiser's suggestion in #1820 (comment), by creating a separate crate for the ast module (rust_python_ast). This will enable us to further split up the ruff crate, as we'll be able to create (e.g.) separate sub-linter crates that have access to these common AST utilities.

This was mostly a straightforward copy (with adjustments to module imports), as the few dependencies that did require modifications were handled in #3366, #3367, and #3368.

[charliermarsh]

We could make this something more general, like ruff_python_syntax, as suggested in the linked issue, which would let us move more code into this crate (e.g., docstrings).

I actually went through with this change misremembering that suggestion. I don't feel strongly either way.

(If we use ruff_python_syntax, we probably want to then scope all this code under ruff_python_syntax::ast.)