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>

.github/workflows/ci.yaml

@@ -259,6 +259,10 @@ jobs:
         uses: taiki-e/install-action@6064345e6658255e90e9500fdf9a06ab77e6909c # v2.57.6
         with:
           tool: cargo-insta
+      - name: "Install uv"
+        uses: astral-sh/setup-uv@e92bafb6253dcd438e0484186d7669ea7a8ca1cc # v6.4.3
+        with:
+          enable-cache: "true"
       - name: ty mdtests (GitHub annotations)
         if: ${{ needs.determine_changes.outputs.ty == 'true' }}
         env:
@@ -317,6 +321,10 @@ jobs:
         uses: taiki-e/install-action@6064345e6658255e90e9500fdf9a06ab77e6909c # v2.57.6
         with:
           tool: cargo-insta
+      - name: "Install uv"
+        uses: astral-sh/setup-uv@e92bafb6253dcd438e0484186d7669ea7a8ca1cc # v6.4.3
+        with:
+          enable-cache: "true"
       - name: "Run tests"
         shell: bash
         env:
@@ -340,6 +348,10 @@ jobs:
         uses: taiki-e/install-action@6064345e6658255e90e9500fdf9a06ab77e6909c # v2.57.6
         with:
           tool: cargo-nextest
+      - name: "Install uv"
+        uses: astral-sh/setup-uv@e92bafb6253dcd438e0484186d7669ea7a8ca1cc # v6.4.3
+        with:
+          enable-cache: "true"
       - name: "Run tests"
         shell: bash
         env: