Add TypeScript config

package.json

@@ -21,6 +21,7 @@
     "build-legacy": "rm -rf build && npm run build-index && rollup --config config/rollup.js && cleancss --source-map src/ol/ol.css -o build/ol.css",
     "copy-css": "cp src/ol/ol.css build/ol/ol.css",
     "transpile": "rm -rf build/ol && mkdir -p build && buble --input src/ol --output build/ol --no modules --sourcemap",
+    "typecheck": "tsc --pretty",
     "apidoc": "jsdoc config/jsdoc/api/index.md -c config/jsdoc/api/conf.json -P package.json -d build/apidoc"
   },
   "main": "index.js",
@@ -74,6 +75,7 @@
     "rollup-plugin-sourcemaps": "0.4.2",
     "rollup-plugin-uglify": "5.0.2",
     "sinon": "^6.0.0",
+    "typescript": "^3.1.0-dev.20180905",
     "uglifyjs-webpack-plugin": "^1.2.5",
     "url-polyfill": "^1.0.13",
     "walk": "^2.3.9",