Allow work stealing schedulers to be added dynamically

[brson]

At the moment, all the schedulers that participate in work stealing (ones that aren't spawned with SingleThreaded mode) are initialized at startup. Servo needs to be able to increase this set dynamically when it spawns SpiderMonkey tasks.

Probably doesn't need to be exposed outside of the unstable rt API's for now.

[alexcrichton]

This will be dealt with in #10965

[alexcrichton]

Closed by #10965 (using the SchedPool type)