Fix exports in package.json (#235)

TypeScript 4.7 introduced the `node16` module resolution. This is more
strict, and more correct than older module resolution options.

According to TypeScript, before this change it is possible to use
`require('ics')`, but not `import 'ics'` from native ESM. This is
because there is an explicit `require` export, that is accompanied by a
`.d.ts` file, but there’s also a default export, which is untyped.

This change fixes it by removing `index.js` altogether. Its only
purpose was to re-export everything from `dist/index.js`. This is now
handled by pointing the main entrypoint and the default export in
`package.json` to `dist/index.js`. The `types` export is necessary,
because the file doesn’t exist in the same place as the dist files.

The `module` option was removed entirely. This is a faux ESM option that
is supposed to point to a module using ESM syntax. It’s pointing to a
CJS file instead, and it would now match the `main` field anyway.

package.json

@@ -3,11 +3,10 @@
   "version": "3.0.1",
   "description": "iCal (ics) file generator",
   "exports": {
-    "require": "./index.js",
+    "types": "./index.d.ts",
     "default": "./dist/index.js"
   },
-  "main": "index.js",
-  "module": "./dist/index.js",
+  "main": "./dist/index.js",
   "types": "index.d.ts",
   "scripts": {
     "start": "mocha --require @babel/register --watch --recursive",