Expose `timeStart/timeEnd/timeAsync` helpers to emitter context

[timotheeguerin]

export function $onEmit(context) {
   context.timeStart("step1")
   // do work
   context.timeEnd("step1");

  context.timeAsync("step2", () => doStep2()); 
}

which then when running tsp compile . --stats would include those steps under the emitter time

probably also need a reportTime with a custom tracking or maybe only that so that it can be included in the result of some emitter API and not have to be forced into passing the context around