Fix import error in CRA environment

Using @planet/maps in a CRA app was causing an import error
since the package.json specifys `type: "module"` the loader
wants this import to end in `.js`.

lib/Map.js

@@ -13,7 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import OLMap from 'ol/Map';
+import OLMap from 'ol/Map.js';
 import {Component, createElement, createRef, forwardRef} from 'react';
 import {any, func, node, object, oneOfType, shape, string} from 'prop-types';
 import {render, updateInstanceFromProps} from '../renderer/render.js';