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):
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
ce4c15dca6