CI: Add GitHub Action build for JavaScript

Uses mymindstorm/setup-emsdk to install Emscripten and set up
caching for Emscripten's generated system libraries.

.github/workflows/javascript_builds.yml

@@ -0,0 +1,76 @@
+name: JavaScript Builds
+on: [push, pull_request]
+
+# Global Cache Settings
+env:
+  SCONS_CACHE_LIMIT: 8192
+  EM_VERSION: latest
+  EM_CACHE_FOLDER: 'emsdk-cache'
+
+jobs:
+  javascript-template:
+    runs-on: "ubuntu-20.04"
+    name: Template (target=release, tools=no)
+
+    steps:
+      - uses: actions/checkout@v2
+
+      # Azure repositories are not reliable, we need to prevent azure giving us packages.
+      - name: Make apt sources.list use the default Ubuntu repositories
+        run: |
+          sudo cp -f misc/ci/sources.list /etc/apt/sources.list
+          sudo apt-get update
+
+      # Upload cache on completion and check it out now
+      # Editing this is pretty dangerous for windows since it can break and needs properly tested with a fresh cache.
+      # Linux with this will work reliably, so not as bad to edit for Linux.
+      - name: Load .scons_cache directory
+        id: javascript-template-cache
+        uses: actions/cache@v2
+        with:
+          path: ${{github.workspace}}/.scons_cache/
+          key: ${{github.job}}-${{github.sha}}
+          restore-keys: |
+            ${{github.job}}-${{github.sha}}
+            ${{github.job}}
+
+      # Additional cache for Emscripten generated system libraries
+      - name: Load Emscripten cache
+        id: javascript-template-emscripten-cache
+        uses: actions/cache@v2
+        with:
+          path: ${{env.EM_CACHE_FOLDER}}
+          key: ${{env.EM_VERSION}}-${{github.job}}
+
+      # Use python 3.x release (works cross platform)
+      - name: Set up Python 3.x
+        uses: actions/setup-python@v2
+        with:
+          # Semantic version range syntax or exact version of a Python version
+          python-version: '3.x'
+          # Optional - x64 or x86 architecture, defaults to x64
+          architecture: 'x64'
+
+      # You can test your matrix by printing the current Python version
+      - name: Configuring Python packages
+        run: |
+          python -c "import sys; print(sys.version)"
+          python -m pip install scons
+          python --version
+          scons --version
+
+      - name: Set up Emscripten latest
+        uses: mymindstorm/setup-emsdk@v6
+        with:
+          version: ${{env.EM_VERSION}}
+          actions-cache-folder: ${{env.EM_CACHE_FOLDER}}
+
+      - name: Verify Emscripten setup
+        run: |
+          emcc -v
+
+      - name: Compilation
+        env:
+          SCONS_CACHE: ${{github.workspace}}/.scons_cache/
+        run: |
+          scons -j2 verbose=yes warnings=all werror=yes platform=javascript target=release tools=no use_closure_compiler=yes