CI: Backport GitHub Actions setup from master

Removes AppVeyor and all Travis jobs but the iOS one, which hasn't been ported
to GitHub Actions yet (should be done soon).

Backports new style scripts from `master` branch too to do the same checks.

.github/workflows/javascript_builds.yml

@@ -0,0 +1,76 @@
+name: JavaScript Builds
+on: [push, pull_request]
+
+# Global Cache Settings
+env:
+  GODOT_BASE_BRANCH: 3.2
+  SCONS_CACHE_LIMIT: 4096
+  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
+      - name: Load .scons_cache directory
+        id: javascript-template-cache
+        uses: actions/cache@v2
+        with:
+          path: ${{github.workspace}}/.scons_cache/
+          key: ${{github.job}}-${{env.GODOT_BASE_BRANCH}}-${{github.ref}}-${{github.sha}}
+          restore-keys: |
+            ${{github.job}}-${{env.GODOT_BASE_BRANCH}}-${{github.ref}}-${{github.sha}}
+            ${{github.job}}-${{env.GODOT_BASE_BRANCH}}-${{github.ref}}
+            ${{github.job}}-${{env.GODOT_BASE_BRANCH}}
+
+      # 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