feat(statusline): vim.diagnostic.status() #33723

Problem:
Not easy to get a status string for diagnostics.

Solution:
- Add vim.diagnostic.status().
- Add it to the default 'statusline'.

test/functional/ui/statusline_spec.lua

@@ -792,6 +792,7 @@ describe('default statusline', function()
       "%{% &showcmdloc == 'statusline' ? '%-10.S ' : '' %}",
       "%{% exists('b:keymap_name') ? '<'..b:keymap_name..'> ' : '' %}",
       "%{% &busy > 0 ? '◐ ' : '' %}",
+      "%(%{luaeval('(pcall(require, ''vim.diagnostic'') and vim.diagnostic.status()) or '''' ')} %)",
       "%{% &ruler ? ( &rulerformat == '' ? '%-14.(%l,%c%V%) %P' : &rulerformat ) : '' %}",
     })