Problem:
checkhealth report sections are not visually separated.
Solution:
Highlight with "reverse".
TODO: migrate checkhealth filetype to use treesitter.
TODO: default :help should also highlight headings more boldy!
This replaces the custom `health{Error,Warning,Success}` highlight
groups with `Diagnostic{Error,Warning,Ok}`, which are defined by
default. Removes the link for `healthHelp`, which was no longer
actually used after #20879.
- If Nvim was just started, don't create a new tab.
- Name the buffer "health://".
- Use "help" syntax instead of "markdown". It fits better, and
eliminates various workarounds.
- Simplfy formatting, avoid visual noise.
- Don't print a "INFO" status, it is noisy.
- Drop the ":" after statuses, they are already UPPERCASE and highlighted.
