Make `Grinder.tasks` a `Map`, remove `Grinder.getTask`

Currently you can access the tasks via `grinder.tasks` or look up an individual task via `grinder.getTask(name)`. But those are disjoint from each other, and accessing just the task names requires something like `grinder.tasks.map((task) => task.name)`.

I think a more intuitive and idiomatic API would be:

`Map get tasks;`

It's a breaking change, but it's unlikely to break many if any folks yet.