Add github actions CI script

Author: marijnh

.github/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: main
+on: [push]
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    name: Build and test
+    steps:
+      - uses: actions/checkout@v1
+
+      - uses: actions/cache@v2
+        with:
+          path: '/home/runner/work/codemirror/codemirror/node_modules'
+          key: ${{ runner.os }}-modules
+
+      - run: npm prepare
+
+      - run: npm test

.travis.yml

@@ -1,6 +1,6 @@
 # CodeMirror
 
-[![Build Status](https://travis-ci.org/codemirror/CodeMirror.svg)](https://travis-ci.org/codemirror/CodeMirror)
+[![Build Status](https://github.com/codemirror/codemirror/workflows/main/badge.svg)](https://github.com/codemirror/codemirror/actions)
 [![NPM version](https://img.shields.io/npm/v/codemirror.svg)](https://www.npmjs.org/package/codemirror)
 
 CodeMirror is a versatile text editor implemented in JavaScript for