Add prototype of ruff format for projects (#6871)

**Summary** Add recursive formatting based on `ruff check` file
discovery for `ruff format`, as a prototype for the formatter alpha.
This allows e.g. `format ../projects/django/`. It's still lacking
support for any settings except line length.

Note just like the existing `ruff format` this will become part of the
production build, i.e. you'll be able to use it - hidden by default and
with a prominent warning - with `ruff format .` after the next release.

Error handling works in my manual tests (the colors do also work):

```
$  target/debug/ruff format scripts/
warning: `ruff format` is a work-in-progress, subject to change at any time, and intended for internal use only.
```
(the above changes `add_rule.py` where we have the wrong bin op
breaking)

```
$ target/debug/ruff format ../projects/django/
warning: `ruff format` is a work-in-progress, subject to change at any time, and intended for internal use only.
Failed to format /home/konsti/projects/django/tests/test_runner_apps/tagged/tests_syntax_error.py: source contains syntax errors: ParseError { error: UnrecognizedToken(Name { name: "syntax_error" }, None), offset: 131, source_path: "<filename>" }
```

```
$ target/debug/ruff format a
warning: `ruff format` is a work-in-progress, subject to change at any time, and intended for internal use only.
Failed to read /home/konsti/ruff/a/d.py: Permission denied (os error 13)
```

**Test Plan** Missing! I'm not sure if it's worth building tests at this
stage or how they should look like.

crates/ruff_python_formatter/README.md

@@ -3,6 +3,15 @@
 The goal of our formatter is to be compatible with Black except for rare edge cases (mostly
 involving comment placement).
 
+You can try an experimental version of the formatter on your project with:
+
+```shell
+cargo run --bin ruff -- format path/to/your/project
+```
+
+Note that currently the only supported option is `line-length` and that both the CLI and the
+formatting are a work-in-progress and will change before the stable release.
+
 ## Dev tools
 
 **Testing your changes** You can use the `ruff_python_formatter` binary to format individual files

crates/ruff_python_formatter/src/lib.rs

@@ -99,9 +99,9 @@ where
 
 #[derive(Error, Debug)]
 pub enum FormatModuleError {
-    #[error("source contains syntax errors (lexer error): {0:?}")]
+    #[error("source contains syntax errors: {0:?}")]
     LexError(LexicalError),
-    #[error("source contains syntax errors (parser error): {0:?}")]
+    #[error("source contains syntax errors: {0:?}")]
     ParseError(ParseError),
     #[error(transparent)]
     FormatError(#[from] FormatError),