add the catch_all binding key

Part of #9963

This adds a new special key `catch_all` that can be used in keybinding
definitions to match any key that is not explicitly bound. For example:
`keybind = catch_all=new_window` (chaos!). 

`catch_all` can be used in combination with modifiers, so if you want to
catch any non-bound key with Ctrl held down, you can do:
`keybind = ctrl+catch_all=new_window`.

`catch_all` can also be used with trigger sequences, so you can do:
`keybind = ctrl+a>catch_all=new_window` to catch any key pressed after
`ctrl+a` that is not explicitly bound and make a new window!

And if you want to remove the catch all binding, it is like any other:
`keybind = catch_all=unbind`.

include/ghostty.h

@@ -317,12 +317,14 @@ typedef struct {
 typedef enum {
   GHOSTTY_TRIGGER_PHYSICAL,
   GHOSTTY_TRIGGER_UNICODE,
+  GHOSTTY_TRIGGER_CATCH_ALL,
 } ghostty_input_trigger_tag_e;
 
 typedef union {
   ghostty_input_key_e translated;
   ghostty_input_key_e physical;
   uint32_t unicode;
+  // catch_all has no payload
 } ghostty_input_trigger_key_u;
 
 typedef struct {