Diaresis

**What problem does this solve?**
I would love it if Harper could be induced to coöperate with my favourite affectation.

**Proposed Solution**
A naïve solution would simply add [these words](https://github.com/RichardLitt/diaeresis-word-list) to the dictionary. Some stemming may be needed, reëntrant itself is not on there although reënter is.

If you can't stand the thought of not scolding the likes of us at least once before we preëmpt the default, then a flag.

**Examples**
> Show, don't tell.

Going over the list I must say, vacuüm is... barbaric. Has anyone ever? Should be vacuoum!

**Component**
- [x] Core engine
- [ ] Plugin/Extension
- [ ] Other: _____

**Additional Context**
It's not an oömlaut, for the record.

Start by locating Harper's core-engine dictionary and stemming entry points, then compare their behavior with the linked diaeresis word list. Clarify whether the accepted approach is dictionary additions, stemming support, or an opt-in flag; done means the listed examples are handled consistently with tests covering them.

Written by the indexing model from the issue text.