Release CI action

Builds the extension for both sites and uploads them as artifacts. This
will make it possible to just release the project and then grab the
ready-to-be-deployed archive for publishing.

This change was made using Cursor. Just a small test run to check how
useful it is for my workflows.

.github/workflows/build-extensions.yml

@@ -0,0 +1,63 @@
+name: Build Extensions
+
+on:
+  push:
+    branches: [ master ]
+
+jobs:
+  build-extensions:
+    runs-on: ubuntu-latest
+    
+    strategy:
+      matrix:
+        site: [furbooru, derpibooru]
+    
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+      
+    - name: Setup Node.js
+      uses: actions/setup-node@v4
+      with:
+        node-version: '20'
+        cache: 'npm'
+        
+    - name: Install dependencies
+      run: npm ci
+      
+    - name: Build extension for ${{ matrix.site }}
+      run: |
+        if [ "${{ matrix.site }}" = "derpibooru" ]; then
+          npm run build:derpibooru
+        else
+          npm run build
+        fi
+        
+    - name: Create extension zip
+      run: |
+        cd build
+        zip -r "../${{ matrix.site }}-tagging-assistant-extension.zip" .
+        
+    - name: Upload extension artifact
+      uses: actions/upload-artifact@v4
+      with:
+        name: ${{ matrix.site }}-tagging-assistant-extension
+        path: ${{ matrix.site }}-tagging-assistant-extension.zip
+        retention-days: 30
+
+  create-release-artifacts:
+    runs-on: ubuntu-latest
+    needs: build-extensions
+    
+    steps:
+    - name: Download all artifacts
+      uses: actions/download-artifact@v4
+      with:
+        path: artifacts
+        
+    - name: Create combined artifact
+      uses: actions/upload-artifact@v4
+      with:
+        name: all-extensions
+        path: artifacts/
+        retention-days: 90