commands.lua: split this script out of console.lua

It was suggested by CogentRedTester in
https://github.com/mpv-player/mpv/pull/10282#issuecomment-1858727729 and
https://github.com/mpv-player/mpv/pull/10282#issuecomment-1858809580 and
by avih that making running commands an mp.input client is a better
architecture.

A practical advantage is that completions are calculated in a different
thread, which prevents hanging when completing slow/network filesystems.

script-binding console/enable becomes script-binding commands/open,
though the console one is kept as an alias. I took the opportunity to
rename this because open makes more sense for a graphical modal, and it
is the word used in mp.input and user-data.

script-message-to console type becomes script-message-to commands type,
though the console one is kept as an alias. It is also changed to
automatically close on submit without having to append '; keypress ESC'
as you don't need to keep the console open after running prefilled
commands.

Also convert to double quotes like other scripts and rename some
inconsistent functions.

player/core.h

@@ -444,7 +444,7 @@ typedef struct MPContext {
 
     struct mp_ipc_ctx *ipc_ctx;
 
-    int64_t builtin_script_ids[7];
+    int64_t builtin_script_ids[8];
 
     mp_mutex abort_lock;