Split off ruff_cli crate from ruff library

This lets you test the ruff linters or use the ruff library
without having to compile the ~100 additional dependencies
that are needed by the CLI.

Because we set the following in the [workspace] section of Cargo.toml:

   default-members = [".", "ruff_cli"]

`cargo run` still runs the CLI and `cargo test` still tests
the code in src/ as well as the code in the new ruff_cli crate.
(But you can now also run `cargo test -p ruff` to only test the linters.)

src/rustpython_helpers.rs

@@ -5,7 +5,7 @@ use rustpython_parser::mode::Mode;
 use rustpython_parser::{lexer, parser};
 
 /// Collect tokens up to and including the first error.
-pub(crate) fn tokenize(contents: &str) -> Vec<LexResult> {
+pub fn tokenize(contents: &str) -> Vec<LexResult> {
     let mut tokens: Vec<LexResult> = vec![];
     for tok in lexer::make_tokenizer(contents) {
         let is_err = tok.is_err();