defaults.lua: add an exit() function

Scripts can terminate execution by setting mp.keep_running = false. Add
an exit() function to wrap setting mp.keep_running and properly expose
this feature. It can be used e.g. by a thumbnail script to spawn workers
with load-script and then let them quit.

It is not added to the mp namespace as mp.exit because that would make
it look like it terminates mpv.

This mirrors the exit() function which already exists in js.

The note in javascript.rst about having to remove key bindings before
exit is not kept because they are actually removed automatically since
bf385e1140 (though it was accurate when the JS backend was developed
before upstreaming it).

player/lua/defaults.lua

@@ -416,6 +416,10 @@ end
 -- used by default event loop (mp_event_loop()) to decide when to quit
 mp.keep_running = true
 
+function _G.exit()
+    mp.keep_running = false
+end
+
 local event_handlers = {}
 
 function mp.register_event(name, cb)
@@ -455,7 +459,7 @@ function mp.unregister_event(cb)
 end
 
 -- default handlers
-mp.register_event("shutdown", function() mp.keep_running = false end)
+mp.register_event("shutdown", exit)
 mp.register_event("client-message", message_dispatch)
 mp.register_event("property-change", property_change)