github actions, run CI on three OS

.github/workflows/ci-build.yml

@@ -0,0 +1,37 @@
+name: CI Build
+on:
+  pull_request:
+    branches: [ 'master' ]
+
+jobs:
+  build:
+    name: CI Build on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 20
+    strategy:
+      matrix:
+        include:
+          - os: ubuntu-latest
+          - os: macos-latest
+          - os: windows-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: coursier/setup-action@v1.2.0-M3
+        apps: sbtn
+      - uses: coursier/cache-action@v6
+
+      - name: Build
+        run: sbtn compile
+
+      - uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cache/scalablytyped/tests/artifacts
+            ~/.cache/scalablytyped/test-artifacts
+          key: ${{ runner.os }}-test-cache-v1
+
+      - name: Build and test
+        run: sbtn test
+
+      - name: scalafmt
+        run: sbtn scalafmtCheck

.github/workflows/release-snapshot.yml

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+# note that the proper release is made manually with the release script because it's
+# a heavy operation (builds all demo repos)
+# that's why we don't trigger on tag
+name: Release snapshot
+on:
+  push:
+    branches: [ 'master' ]
+jobs:
+  release-snapshot:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: coursier/setup-action@v1.2.0-M3
+        apps: sbtn
+      - uses: coursier/cache-action@v6
+
+      - name: Build native image (non-windows)
+        run: sbtn test scalafmtCheck
+
+      - name: publish snapshot
+        run: sbt ci-release