convert export default object to named exports

Note: Normally I would use export function ..., but since we also have
some export let xx = createUtilityPlugin in this file, it means that all
the `export function` declarations would be hoisted and therefore won't
have the correct order anymore.

To fix this, I used `export let xx = () => {}` instead of the usual
`export function xx() {}`

Author: RobinMalfait

scripts/create-plugin-list.js

@@ -1,4 +1,4 @@
-import corePlugins from '../src/corePlugins'
+import * as corePlugins from '../src/corePlugins'
 import fs from 'fs'
 import path from 'path'