feat(highlight): add DiagnosticOk (and associated) highlight groups (#21286)

The existing groups, Error, Hint, Info, Warn cover many use cases, but
neglect the occasion where a diagnostic message should communicate a
non-informative (not a Hint or Info) event. DiagnosticOk covers this
with a generic green colorscheme.

test/functional/lua/diagnostic_spec.lua

@@ -90,20 +90,25 @@ describe('vim.diagnostic', function()
       'DiagnosticFloatingError',
       'DiagnosticFloatingHint',
       'DiagnosticFloatingInfo',
+      'DiagnosticFloatingOk',
       'DiagnosticFloatingWarn',
       'DiagnosticHint',
       'DiagnosticInfo',
+      'DiagnosticOk',
       'DiagnosticSignError',
       'DiagnosticSignHint',
       'DiagnosticSignInfo',
+      'DiagnosticSignOk',
       'DiagnosticSignWarn',
       'DiagnosticUnderlineError',
       'DiagnosticUnderlineHint',
       'DiagnosticUnderlineInfo',
+      'DiagnosticUnderlineOk',
       'DiagnosticUnderlineWarn',
       'DiagnosticVirtualTextError',
       'DiagnosticVirtualTextHint',
       'DiagnosticVirtualTextInfo',
+      'DiagnosticVirtualTextOk',
       'DiagnosticVirtualTextWarn',
       'DiagnosticWarn',
     }, exec_lua([[return vim.fn.getcompletion('Diagnostic', 'highlight')]]))