chore(ci): set up GitHub Actions workflow

This should completely replace our Travis, AppVeyor, and Azure Pipelines
configs, since GH Actions can run across target OSes straightforwardly.

Resolves #215

Author: chriskrycho

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+jobs:
+  linting:
+    name: Linting
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: 10.x
+
+      - run: yarn install --frozen-lockfile --non-interactive
+      - run: yarn lint:js
+
+  tests:
+    name: 'Tests: ${{ matrix.os }}'
+    runs-on: '${{ matrix.os }}-latest'
+
+    strategy:
+      matrix:
+        os: [ubuntu, macOS, windows]
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: 10.x
+
+      - run: yarn install --frozen-lockfile --non-interactive
+      - run: yarn test