Also implicitly fixes memory leak when mp.set_property_native was used,
because the cleanup did not expect more allocations from the accidental
use of mpv_get_property.
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.