[REVIEW ONLY] Add Find whole word functionality

This PR implements https://trello.com/c/xabJJVcu/1078-find-replace-whole-word-toggle
It has not yet an icon, but you can use it.

Tasks to be done:
* [ ] Unit tests
* [ ] Only allow either `Whole word` or `Regular expression` toggle to be active at once
* [ ] Icon (@larz0?)
* ~~Scoping `$()` searches to current ModalBar (#7512)~~

The things I tested worked the same as in Sublime Text, but I found one scenario where it didn't work the way it works in Visual Studio:
1. Open a file with the content `if ("\\b")`
2. Find (whole word, of course) `\\b`

Nor Brackets neither Sublime find anything, but VS does. We'd probably need regexp lookbefores in order to fix this, but those are not supported in JavaScript.