Add GitHub Actions workflow to publish to GitHub Pages

Taken verbatim from https://github.com/endlessm/moddable-platformer.

Fixes #2

.github/dependabot.yml

@@ -0,0 +1,11 @@
+# https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot
+# Set update schedule for GitHub Actions
+
+version: 2
+updates:
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      # Check for updates to GitHub Actions every week
+      interval: "weekly"

.github/workflows/github-pages.yml

@@ -0,0 +1,116 @@
+name: "Publish to GitHub Pages"
+on:
+  workflow_dispatch:
+  release:
+    types:
+      - published
+
+# Cancel any ongoing previous run if the job is re-triggered
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+env:
+  GODOT_VERSION: 4.3
+
+jobs:
+  check:
+    name: Check if GitHub Pages is enabled
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check
+        id: check
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          if gh api "repos/${{ github.repository }}/pages" | jq --exit-status '.build_type == "workflow"'
+          then
+              echo "enabled=true" >> "$GITHUB_OUTPUT"
+          else
+              echo "# Not published to GitHub Pages" >> "$GITHUB_STEP_SUMMARY"
+              echo "" >> "$GITHUB_STEP_SUMMARY"
+              echo -n "Check that Pages is enabled, with the source set to GitHub Actions, in the " >> "$GITHUB_STEP_SUMMARY"
+              echo "[repository settings](https://github.com/${{ github.repository }}/settings/pages)." >> "$GITHUB_STEP_SUMMARY"
+          fi
+    outputs:
+      enabled: ${{ steps.check.outputs.enabled }}
+
+  build:
+    name: Build for web
+    needs:
+      - check
+    if: ${{ needs.check.outputs.enabled }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          lfs: true
+
+      - name: Cache Godot Engine downloads
+        id: cache-godot
+        uses: actions/cache@v4
+        with:
+          path: |
+            build/godot
+            build/._sc_
+            build/editor_data/export_templates/${{ env.GODOT_VERSION }}.stable
+          key: godot-${{ env.GODOT_VERSION }}
+
+      - name: Download Godot Engine from GitHub release
+        id: download
+        if: steps.cache-godot.outputs.cache-hit != 'true'
+        run: |
+          mkdir -p build && cd build
+
+          # Download Godot Engine itself
+          wget https://github.com/godotengine/godot/releases/download/${GODOT_VERSION}-stable/Godot_v${GODOT_VERSION}-stable_linux.x86_64.zip && \
+          unzip Godot_v${GODOT_VERSION}-stable_linux.x86_64.zip && \
+          mv Godot_v${GODOT_VERSION}-stable_linux.x86_64 godot
+
+          # Download export templates
+          mkdir -p editor_data/export_templates
+          wget https://github.com/godotengine/godot/releases/download/${GODOT_VERSION}-stable/Godot_v${GODOT_VERSION}-stable_export_templates.tpz && \
+          unzip Godot_v${GODOT_VERSION}-stable_export_templates.tpz && \
+          mv templates editor_data/export_templates/${GODOT_VERSION}.stable
+
+          # Tell Godot Engine to run in "self-contained" mode so it looks for
+          # templates here instead of in ~/.local/share/godot/
+          touch ._sc_
+
+      - name: Web Build
+        run: |
+          mkdir -v -p build/web && cd build
+
+          # Note that the export path can be confusing; it's relative to the
+          # Godot project path, NOT necessarily the current directory or Godot
+          # binary location
+          ./godot --headless --verbose --path ../ --export-release "Web" ./build/web/index.html
+
+      - name: Upload Artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          name: web
+          path: build/web
+
+  publish:
+    name: Publish to GitHub Pages
+    needs:
+      - build
+    runs-on: ubuntu-latest
+    steps:
+      - id: deploy
+        name: Deploy to GitHub Pages
+        uses: actions/deploy-pages@v4
+        with:
+          artifact_name: web
+
+      - name: Show URL in summary
+        if: ${{ steps.deploy.outcome == 'success' }}
+        run: |
+          echo "Game published to: <${{ steps.deploy.outputs.page_url }}>" >> "$GITHUB_STEP_SUMMARY"