Add Runic using GHA (#2268)

[Runic](https://github.com/fredrikekre/Runic.jl) is a newish Julia formatter, that JuliaGPU has been adopting.
A particularly nice feature is it's ability to format only ranges,
and thus we can use a GHA to format Enzyme over-time.

Using [pre-commit](https://pre-commit.com/) one can also easily install
a git pre commit hook.

.github/workflows/Format.yml

@@ -0,0 +1,55 @@
+name: 'Format'
+
+on:
+  pull_request_target:
+    paths: ['**/*.jl']
+    types: [opened, synchronize, reopened, ready_for_review]
+
+permissions:
+  contents: read
+
+  # needed for julia-actions/cache to delete old caches
+  actions: write
+
+  # needed for googleapis/code-suggester
+  pull-requests: write
+
+jobs:
+  runic:
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.draft == false
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{github.event.pull_request.head.ref}}
+          repository: ${{github.event.pull_request.head.repo.full_name}}
+          fetch-depth: 0
+
+      - name: Setup Julia
+        uses: julia-actions/setup-julia@v2
+        with:
+          version: '1'
+          arch: 'x64'
+      - uses: julia-actions/cache@v2
+
+      - name: Install Runic
+        run: |
+          julia --project=@runic -e 'using Pkg; Pkg.add("Runic")'
+          curl -o git-runic https://raw.githubusercontent.com/fredrikekre/Runic.jl/master/bin/git-runic
+          chmod +x git-runic
+          sudo mv git-runic /usr/local/bin
+
+      - name: Run Runic
+        run: |
+          set +e
+          git runic origin/master
+          [ $? -eq 2 ] && exit 1 || exit 0
+
+      - name: Suggest changes
+        uses: googleapis/code-suggester@v2
+        env:
+          ACCESS_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          command: review
+          pull_number: ${{ github.event.pull_request.number }}
+          git_dir: '.'

.pre-commit-config.yaml

@@ -0,0 +1,5 @@
+repos:
+  - repo: https://github.com/fredrikekre/runic-pre-commit
+    rev: v1.0.0
+    hooks:
+      - id: runic