ci: introduce labeler workflow

Automatically tag PRs depending on touched files.

.github/labeler.yml

@@ -0,0 +1,4 @@
+"neovim":
+  - modules/programs/neovim.nix
+  - tests/modules/programs/neovim/**/*
+

.github/workflows/labeler.yml

@@ -0,0 +1,24 @@
+name: "Label PR"
+
+on:
+  - pull_request_target
+
+# WARNING:
+# When extending this action, be aware that $GITHUB_TOKEN allows some write
+# access to the GitHub API. This means that it should not evaluate user input in
+# a way that allows code injection.
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  labels:
+    runs-on: ubuntu-latest
+    if: github.repository_owner == 'nix-community'
+    steps:
+      - uses: actions/labeler@v4
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          sync-labels: true
+