This reverts commit 07406331.

Use `Array<T | U>` when dealing with union types instead of  `(T | U)[]`

ES6 promises like using undefined as their first argument instead of null. Both should behave the same

Try to better support handle of unicode in markdown by using a word definition that use unicode character class

Fixes #15177

This reverts commit d28f895d.

This reverts commit f6cff39c.

related to #63897

Fixes #64216