feat(diagnostic): allow retrieving current diagnostic config

runtime/doc/diagnostic.txt

@@ -334,8 +334,9 @@ config({opts}, {namespace})                          *vim.diagnostic.config()*
                       that returns any of the above.
 
                 Parameters: ~
-                    {opts}       table Configuration table with the following
+                    {opts}       table|nil When omitted or "nil", retrieve the
-                                 keys:
+                                 current configuration. Otherwise, a
+                                 configuration table with the following keys:
                                  • underline: (default true) Use underline for
                                    diagnostics. Options:
                                    • severity: Only underline diagnostics

runtime/lua/vim/diagnostic.lua

@@ -552,7 +552,8 @@ end
 ---         - `table`: Enable this feature with overrides. Use an empty table to use default values.
 ---         - `function`: Function with signature (namespace, bufnr) that returns any of the above.
 ---
----@param opts table Configuration table with the following keys:
+---@param opts table|nil When omitted or "nil", retrieve the current configuration. Otherwise, a
+---                      configuration table with the following keys:
 ---       - underline: (default true) Use underline for diagnostics. Options:
 ---                    * severity: Only underline diagnostics matching the given severity
 ---                    |diagnostic-severity|
@@ -599,7 +600,7 @@ end
 ---                            global diagnostic options.
 function M.config(opts, namespace)
   vim.validate {
-    opts = { opts, 't' },
+    opts = { opts, 't', true },
     namespace = { namespace, 'n', true },
   }
 
@@ -611,6 +612,11 @@ function M.config(opts, namespace)
     t = global_diagnostic_options
   end
 
+  if not opts then
+    -- Return current config
+    return vim.deepcopy(t)
+  end
+
   for k, v in pairs(opts) do
     t[k] = v
   end