Editorial: modernize timers spec

In addition to general updates to use modern terminology and improve precision, this:

* Moves the Web IDL conversion note and example to after the algorithm.
* Removes the (exported) "list of active timers" concept since it's not something other specs should be messing with.
* Renames the return value of setTimeout()/setInterval() and the argument to clearTimeout()/clearInterval() from "handle" to "id", since having both "handle" and "handler" was confusing.