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.

runtime/doc/diagnostic.txt

@@ -206,6 +206,11 @@ DiagnosticInfo
 
                                                         *hl-DiagnosticHint*
 DiagnosticHint
+    Used as the base highlight group.
+    Other Diagnostic highlights link to this by default (except Underline)
+
+                                                        *hl-DiagnosticOk*
+DiagnosticOk
     Used as the base highlight group.
     Other Diagnostic highlights link to this by default (except Underline)
 
@@ -225,6 +230,10 @@ DiagnosticVirtualTextInfo
 DiagnosticVirtualTextHint
     Used for "Hint" diagnostic virtual text.
 
+                                                *hl-DiagnosticVirtualTextOk*
+DiagnosticVirtualTextOk
+    Used for "Ok" diagnostic virtual text.
+
                                                 *hl-DiagnosticUnderlineError*
 DiagnosticUnderlineError
     Used to underline "Error" diagnostics.
@@ -241,6 +250,10 @@ DiagnosticUnderlineInfo
 DiagnosticUnderlineHint
     Used to underline "Hint" diagnostics.
 
+                                                *hl-DiagnosticUnderlineOk*
+DiagnosticUnderlineOk
+    Used to underline "Ok" diagnostics.
+
                                                 *hl-DiagnosticFloatingError*
 DiagnosticFloatingError
     Used to color "Error" diagnostic messages in diagnostics float.
@@ -258,6 +271,10 @@ DiagnosticFloatingInfo
 DiagnosticFloatingHint
     Used to color "Hint" diagnostic messages in diagnostics float.
 
+                                                *hl-DiagnosticFloatingOk*
+DiagnosticFloatingOk
+    Used to color "Ok" diagnostic messages in diagnostics float.
+
                                                 *hl-DiagnosticSignError*
 DiagnosticSignError
     Used for "Error" signs in sign column.
@@ -274,6 +291,10 @@ DiagnosticSignInfo
 DiagnosticSignHint
     Used for "Hint" signs in sign column.
 
+                                                *hl-DiagnosticSignOk*
+DiagnosticSignOk
+    Used for "Ok" signs in sign column.
+
 ==============================================================================
 SIGNS                                                   *diagnostic-signs*