fix: add types location for cjs/src/

Now we're publishing a `"main"`, TypeScript is using that rather than the
`"exports"` map. It points to the CJS published version, located in cjs/src
in the bundle. This adds a descriptor that tells TypeScript to look for types
for that location in the types/, which should be the same as to what it finds
in the export map.

Ref: #64 (comment)

package.json

@@ -121,6 +121,9 @@
     "*": {
       "*": [
         "types/*"
+      ],
+      "cjs/src/*": [
+        "types/*"
       ]
     }
   }