[REVIEW ONLY] Add Find Whole Word functionality

New version of #7604.

This PR implements https://trello.com/c/xabJJVcu/1078-find-replace-whole-word-toggle.
It doesn't have an icon yet, but you can use it without restrictions.

To be done:
- [ ] Unit tests
- [x] Only allow either Whole word or Regular expression toggle to be active at once
- [x] Icon (@larz0?)

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 ("abc")`
2. Find Whole Word: `abc`

Neither Brackets nor Sublime find anything, but VS does. We'd probably need regexp lookbefores in order to fix this, but those are not supported in JavaScript.