feat(cli): add basis for new "emacs-builder" CLI tool written in Go

This will eventually replace the build-emacs-for-macos script that's
written in Ruby. The migration of functionality will happen bit by bit
over time.

.github/workflows/ci.yml

@@ -0,0 +1,52 @@
+---
+name: CI
+on: [push]
+
+jobs:
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: golangci-lint
+        uses: golangci/golangci-lint-action@v2
+        with:
+          version: v1.40
+        env:
+          VERBOSE: "true"
+
+  tidy:
+    name: Tidy
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-go@v2
+        with:
+          go-version: 1.16
+      - uses: actions/cache@v2
+        with:
+          path: ~/go/pkg/mod
+          key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
+          restore-keys: |
+            ${{ runner.os }}-go-
+      - name: Check if mods are tidy
+        run: make check-tidy
+
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-go@v2
+        with:
+          go-version: 1.16
+      - uses: actions/cache@v2
+        with:
+          path: ~/go/pkg/mod
+          key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
+          restore-keys: |
+            ${{ runner.os }}-go-
+      - name: Run tests
+        run: make test
+        env:
+          VERBOSE: "true"