1. Included unit tests
2. I tested the chat mentions experience
3. I did some perf testing:
In combination with the following diff, I used [this patch](https://gist.github.com/Ashoat/fc1c91a61009de0e9959527454be8236) to test performance before and after this change. I made sure I had at least three samples of each scenario. Will also link my [messy Gist of results](https://gist.github.com/Ashoat/b871afaaaee10b435b8676175d120d53), but it's not really interpretable by anyone other than me.
Here's the relevant portion:
```
BEFORE
LOG useChatMentionSearchIndex took 1801ms
LOG useChatMentionSearchIndex took 1748ms
LOG useChatMentionSearchIndex took 1730ms
LOG useChatMentionSearchIndex took 1831ms
AVERAGE 1777.5ms
JUST DEDUP (parent diff)
LOG useChatMentionSearchIndex took 1027ms
LOG useChatMentionSearchIndex took 949ms
LOG useChatMentionSearchIndex took 957ms
AVERAGE 977.7ms
DEDUP + RADIX TREE
LOG useChatMentionSearchIndex took 643ms
LOG useChatMentionSearchIndex took 629ms
LOG useChatMentionSearchIndex took 651ms
LOG useChatMentionSearchIndex took 609ms
AVERAGE 633ms
JUST RADIX TREE
LOG useChatMentionSearchIndex took 1394ms
LOG useChatMentionSearchIndex took 1468ms
LOG useChatMentionSearchIndex took 1511ms
LOG useChatMentionSearchIndex took 1492ms
LOG useChatMentionSearchIndex took 1397ms
AVERAGE 1452.4ms
```