fixes https://github.com/zed-industries/community/issues/48

Release notes
- Added wrap guides and two associated language settings:
`"show_wrap_guides": bool` and `"wrap_guides": [..]`. The first controls
whether wrap guides are shown when `"soft_wrap":
"preferred_line_length"` is enabled and the second allows Zed to show
additional wrap guides at whichever column index you prefer.

Here's a screenshot of Zed with wrap guides at 60 and 90, and soft wrap
active with a preferred_line_length of 80:

<img width="956" alt="Screenshot 2023-07-20 at 4 42 11 PM"
src="https://github.com/zed-industries/zed/assets/2280405/48f36be1-3bdc-48eb-bfca-e61fcfd6dbc2">