defaults.lua: support custom arg in mp.add_key_binding()

The information is available in the table.

DOCS/man/lua.rst

@@ -316,8 +316,8 @@ The ``mp`` module is preloaded, although it can be loaded manually with
     or pass the ``fn`` argument in place of the name. The latter is not
     recommended and is handled for compatibility only.)
 
-    The last argument is used for optional flags. This is a table, which can
-    have the following entries:
+    The ``flags`` argument is used for optional parameters. This is a table,
+    which can have the following entries:
 
         ``repeatable``
             If set to ``true``, enables key repeat for this specific binding.
@@ -361,6 +361,11 @@ The ``mp`` module is preloaded, although it can be loaded manually with
                     The scale of the key, such as the ones produced by ``WHEEL_*``
                     keys. The scale is 1 if the key is nonscalable.
 
+                ``arg``
+                    User-provided string in the ``arg`` argument in the
+                    ``script-binding`` command if the key binding is invoked
+                    by that command.
+
     Internally, key bindings are dispatched via the ``script-message-to`` or
     ``script-binding`` input commands and ``mp.register_script_message``.
 

player/lua/defaults.lua

@@ -60,10 +60,10 @@ local function reserve_binding()
     return "__keybinding" .. tostring(message_id)
 end
 
-local function dispatch_key_binding(name, state, key_name, key_text, scale)
+local function dispatch_key_binding(name, state, key_name, key_text, scale, arg)
     local fn = dispatch_key_bindings[name]
     if fn then
-        fn(name, state, key_name, key_text, scale)
+        fn(name, state, key_name, key_text, scale, arg)
     end
 end
 
@@ -201,7 +201,7 @@ local function add_binding(attrs, key, name, fn, rp)
             ["r"] = "repeat",
             ["p"] = "press",
         }
-        key_cb = function(_, state, key_name, key_text, scale)
+        key_cb = function(_, state, key_name, key_text, scale, arg)
             if key_text == "" then
                 key_text = nil
             end
@@ -212,6 +212,7 @@ local function add_binding(attrs, key, name, fn, rp)
                 key_name = key_name,
                 key_text = key_text,
                 scale = tonumber(scale),
+                arg = arg,
             })
         end
         msg_cb = function()