Internal/add gh actions and tests (#3)

* add gh actions and templates
* add show-versions in Makefile
* force go 1.20
* added test facilities

.github/workflows/ci.yaml

@@ -0,0 +1,36 @@
+name: build-and-test-rpn
+on: [push, pull_request]
+jobs:
+  build:
+    strategy:
+      matrix:
+        version: [1.21]
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    name: Build
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Set up Go
+      uses: actions/setup-go@v3
+      with:
+        go-version: ${{ matrix.version }}
+      id: go
+
+    - name: checkout
+      uses: actions/checkout@v3
+
+    - name: build
+      run: go build
+
+    - name: test
+      run: make test
+
+  golangci:
+    name: lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/setup-go@v3
+        with:
+          go-version: 1.21
+      - uses: actions/checkout@v3
+      - name: golangci-lint
+        uses: golangci/golangci-lint-action@v3