js: add javascript scripting support using MuJS

Implements JS with almost identical API to the Lua support.

Key differences from Lua:
- The global mp, mp.msg and mp.utils are always available.
- Instead of returning x, error, return x and expose mp.last_error().
- Timers are JS standard set/clear Timeout/Interval.
- Supports CommonJS modules/require.
- Added at mp.utils: getenv, read_file, write_file and few more.
- Global print and dump (expand objects) functions.
- mp.options currently not supported.

See DOCS/man/javascript.rst for more details.

DOCS/man/mpv.rst

@@ -847,6 +847,8 @@ works like in older mpv releases. The profiles are currently defined as follows:
 
 .. include:: lua.rst
 
+.. include:: javascript.rst
+
 .. include:: ipc.rst
 
 .. include:: changes.rst