Add workflow for automated releases.

When a new version tag is pushed, the workflow automatically runs tests
(currently only on linux), produces an executable for each target
system, creates a draft release for the pushed tag, and uploads each
generated executable to the release.

Afterwards, the only manual work required is filling out a description,
then publishing the draft.

.github/workflows/release.yml

@@ -0,0 +1,114 @@
+name: Create draft release
+
+on:
+  push:
+    tags:
+      - 'v*.*.*'
+
+env:
+  PROJECT   : 'rbxmk'  # Name of project/executable.
+  PROGRAM   : '.'      # Location of program.
+  GOVERSION : 1.15     # Version of Go to compile with.
+  DIST      : './dist' # Scratch directory for building executables.
+
+jobs:
+
+  test:
+    name: Run tests
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Set up Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: ${{env.GOVERSION}}
+
+      - name: Run tests
+        run: go test -v ${{env.PROGRAM}}
+
+  build:
+    name: Build executables
+    needs: test
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include:
+          - {os: 'windows' , arch: 'amd64' , output: './dist/playground.exe'}
+          - {os: 'windows' , arch: '386'   , output: './dist/playground.exe'}
+          - {os: 'darwin'  , arch: 'amd64' , output: './dist/playground'}
+          - {os: 'linux'   , arch: '386'   , output: './dist/playground'}
+          - {os: 'linux'   , arch: 'amd64' , output: './dist/playground'}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Set up Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: ${{env.GOVERSION}}
+
+      - name: Set version variable
+        run: echo VERSION=${GITHUB_REF#refs/tags/} >> $GITHUB_ENV
+
+      - name: Make build directory
+        run: mkdir ${{env.DIST}}
+
+      - name: Build executable
+        env:
+          GOOS: ${{matrix.os}}
+          GOARCH: ${{matrix.arch}}
+          OUTPUT: ${{matrix.output}}
+        run: go build -v -o $OUTPUT ${{env.PROGRAM}}
+
+      - name: Create archive
+        id: archive
+        env:
+          GOOS: ${{matrix.os}}
+          GOARCH: ${{matrix.arch}}
+          OUTPUT: ${{matrix.output}}
+        run: |
+          NAME=$PROJECT-$VERSION-$GOOS-$GOARCH
+          ARCHIVE=${{env.DIST}}/$NAME.zip
+          zip --junk-paths $ARCHIVE $OUTPUT
+          echo ::set-output name=name::$NAME
+          echo ::set-output name=path::$ARCHIVE
+
+      - name: Upload executable
+        uses: actions/upload-artifact@v2
+        with:
+          name: ${{steps.archive.outputs.name}}
+          path: ${{steps.archive.outputs.path}}
+          if-no-files-found: error
+          retention-days: 1
+
+  release:
+    name: Create release
+    needs: [test, build]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set version variable
+        run: echo VERSION=${GITHUB_REF#refs/tags/} >> $GITHUB_ENV
+
+      - name: Download all archives
+        id: download
+        uses: actions/download-artifact@v2
+
+      - name: Move files
+        run: |
+          mkdir files
+          mv $(find ${{steps.download.outputs.download-path}} -iname *.zip) files
+
+      - name: Check files
+        run: find .
+
+      - name: Create release
+        uses: softprops/action-gh-release@v1
+        env:
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
+        with:
+          name: ${{env.PROJECT}} ${{env.VERSION}}
+          body: TODO
+          draft: true
+          files: files/*.zip