Fix types paths in exports map (#72)

The types conditions pointed at index.d.js, which does not exist. tsc
strips the .js extension and finds index.d.ts beside it, so consumers
were unaffected, but tools that read exports literally flag it.

Author: microbit-matt-hillsdon

package.json

@@ -14,11 +14,11 @@
   "exports": {
     ".": {
       "import": {
-        "types": "./dist/esm/index.d.js",
+        "types": "./dist/esm/index.d.ts",
         "default": "./dist/esm/index.js"
       },
       "require": {
-        "types": "./dist/cjs/index.d.js",
+        "types": "./dist/cjs/index.d.ts",
         "default": "./dist/cjs/index.js"
       }
     }