[WIP] setup GitHub Actions build and test steps

WORK IN PROGRESS

We define GitHub Actions YAML which runs a simplified version of
the BuildScalarFor*.{bat,sh} and RunUnitTests.{bat,sh} scripts
under Scripts/.

One difference is that we use a Release build configuration
instead of using a Build configuration as our default.

We also do not define the ScalarVersion MSBuild property and
rely on what is set in Directory.Build.props.

h/t kyle-rader

.github/workflows/continuous-integration.yml

@@ -0,0 +1,38 @@
+name: Scalar
+
+on:
+  push:
+    branches: [ main, linux-initial-trial ]
+  pull_request:
+    branches: [ main ]
+
+env:
+  DOTNET_NOLOGO: 1
+
+jobs:
+  validate_scalar:
+    name: "CI"
+
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-16.04, ubuntu-18.04, ubuntu-20.04, windows-2019, macos-10.15]
+
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0 # Indicate full history so Nerdbank.GitVersioning works.
+
+    - name: Setup .NET Core
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.1.302
+
+    - name: Install dependencies
+      run: dotnet restore
+
+    - name: Build
+      run: dotnet build --configuration Release --no-restore
+
+    - name: Test
+      run: dotnet test --no-restore