Add extensions to local imports of files in types

Closes GH-14.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: remcohaszing

lib/index.js

@@ -10,10 +10,10 @@
  * @typedef {string|number|null|undefined} XPrimitiveChild
  * @typedef {Array<Node|XPrimitiveChild>} XArrayChild
  * @typedef {Node|XPrimitiveChild|XArrayChild} XChild
- * @typedef {import('./jsx-classic').Element} x.JSX.Element
- * @typedef {import('./jsx-classic').IntrinsicAttributes} x.JSX.IntrinsicAttributes
- * @typedef {import('./jsx-classic').IntrinsicElements} x.JSX.IntrinsicElements
- * @typedef {import('./jsx-classic').ElementChildrenAttribute} x.JSX.ElementChildrenAttribute
+ * @typedef {import('./jsx-classic.js').Element} x.JSX.Element
+ * @typedef {import('./jsx-classic.js').IntrinsicAttributes} x.JSX.IntrinsicAttributes
+ * @typedef {import('./jsx-classic.js').IntrinsicElements} x.JSX.IntrinsicElements
+ * @typedef {import('./jsx-classic.js').ElementChildrenAttribute} x.JSX.ElementChildrenAttribute
  */
 
 /**