Truncate extension descriptions

This is for #8216.

- [ ] Code to toggle between original and truncated description
- [x] Visual review
- [ ] Code review
I don't know much about JSDoc, but you should probably comment the constant.
[Example](https://github.com/adobe/brackets/blob/47292b3400e432a80ad370245986556c0c0c8a03/src/search/SearchModel.js#L44)