Add support for using uv as an alternative formatter backend (#19665)

This adds a new `backend: internal | uv` option to the LSP
`FormatOptions` allowing users to perform document and range formatting
operations though uv. The idea here is to prototype a solution for users
to transition to a `uv format` command without encountering version
mismatches (and consequently, formatting differences) between the LSP's
version of `ruff` and uv's version of `ruff`.

The primarily alternative to this would be to use uv to discover the
`ruff` version used to start the LSP in the first place. However, this
would increase the scope of a minimal `uv format` command beyond "run a
formatter", and raise larger questions about how uv should be used to
coordinate toolchain discovery. I think those are good things to
explore, but I'm hesitant to let them block a `uv format`
implementation. Another downside of using uv to discover `ruff`, is that
it needs to be implemented _outside_ the LSP; e.g., we'd need to change
the instructions on how to run the LSP and implement it in each editor
integration, like the VS Code plugin.

---------

Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>

crates/ruff_formatter/src/lib.rs

@@ -81,14 +81,19 @@ impl IndentStyle {
     pub const fn is_space(&self) -> bool {
         matches!(self, IndentStyle::Space)
     }
+
+    /// Returns the string representation of the indent style.
+    pub const fn as_str(&self) -> &'static str {
+        match self {
+            IndentStyle::Tab => "tab",
+            IndentStyle::Space => "space",
+        }
+    }
 }
 
 impl std::fmt::Display for IndentStyle {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match self {
-            IndentStyle::Tab => std::write!(f, "tab"),
-            IndentStyle::Space => std::write!(f, "space"),
-        }
+        f.write_str(self.as_str())
     }
 }
 

crates/ruff_formatter/src/printer/printer_options/mod.rs

@@ -139,4 +139,16 @@ impl LineEnding {
             LineEnding::CarriageReturn => "\r",
         }
     }
+
+    /// Returns the string used to configure this line ending.
+    ///
+    /// See [`LineEnding::as_str`] for the actual string representation of the line ending.
+    #[inline]
+    pub const fn as_setting_str(&self) -> &'static str {
+        match self {
+            LineEnding::LineFeed => "lf",
+            LineEnding::CarriageReturnLineFeed => "crlf",
+            LineEnding::CarriageReturn => "cr",
+        }
+    }
 }