This PR implements support for displaying diagnostics in the scrollbar,
similar to what is already done for search results, symbols, git diff,
...

For example, changing a field name (`text`) without changing the
references looks like this in `buffer.rs` (note the red lines in the
scrollbar):

![image](https://github.com/zed-industries/zed/assets/53836821/c46f0d55-32e3-4334-8ad7-66d1578d5725)

As you can see, the errors, warnings, ... are displayed in the scroll
bar, which helps to identify possible problems with the current file.

Relevant issues: #4866, #6819

Release Notes:

- Added diagnostic indicators to the scrollbar