js: implement mp.msg.trace()

To match the new Lua helper introduced in
1afdeee1ad

Add documentation for both.

DOCS/man/javascript.rst

@@ -164,6 +164,8 @@ Otherwise, where the Lua APIs return ``nil`` on error, JS returns ``undefined``.
 
 ``mp.msg.debug(...)``
 
+``mp.msg.trace(...)``
+
 ``mp.utils.getcwd()`` (LE)
 
 ``mp.utils.readdir(path [, filter])`` (LE)

DOCS/man/lua.rst

@@ -487,16 +487,17 @@ with ``require 'mp.msg'``.
 
 ``msg.log(level, ...)``
     The level parameter is the message priority. It's a string and one of
-    ``fatal``, ``error``, ``warn``, ``info``, ``v``, ``debug``. The user's
-    settings will determine which of these messages will be visible. Normally,
-    all messages are visible, except ``v`` and ``debug``.
+    ``fatal``, ``error``, ``warn``, ``info``, ``v``, ``debug``, ``trace``. The
+    user's settings will determine which of these messages will be
+    visible. Normally, all messages are visible, except ``v``, ``debug`` and
+    ``trace``.
 
     The parameters after that are all converted to strings. Spaces are inserted
     to separate multiple parameters.
 
     You don't need to add newlines.
 
-``msg.fatal(...)``, ``msg.error(...)``, ``msg.warn(...)``, ``msg.info(...)``, ``msg.verbose(...)``, ``msg.debug(...)``
+``msg.fatal(...)``, ``msg.error(...)``, ``msg.warn(...)``, ``msg.info(...)``, ``msg.verbose(...)``, ``msg.debug(...)``, ``msg.trace(...)``
     All of these are shortcuts and equivalent to the corresponding
     ``msg.log(level, ...)`` call.
 

player/javascript/defaults.js

@@ -7,7 +7,7 @@
 
 mp.msg = { log: mp.log };
 mp.msg.verbose = mp.log.bind(null, "v");
-var levels = ["fatal", "error", "warn", "info", "debug"];
+var levels = ["fatal", "error", "warn", "info", "debug", "trace"];
 levels.forEach(function(l) { mp.msg[l] = mp.log.bind(null, l) });
 
 // same as {} but without inherited stuff, e.g. o["toString"] doesn't exist.