Clean up Tasks System

[tsmaeder]

Currently, our tasks subsystem is a bit of a mess: there are a couple of disconnects I'd like to clean up:

1. Structured vs. unstructured
   In some places, we use structure values (task, which references a type object, an execution object, etc) vs flat representation in DTO's, for example.
2. Naming
   task.type, taskType, etc. Also "executions" is an overloaded term that could use some clarification
3. Task configuration typing is kind of weird: Look at the type hierarchy of 'TaskCustomization' to see the problem: some fields which are defined are missing (scope, source) or strangely defined (_source, _scope), etc. I think a lot of it comes from the fact that we're treating the content of the tasks.json files as kind of the same thing as the tasks in the system (typing-wise). Maybe cleanly separating those two would clarify the picture.

In the past, our task system diverged from what vscode has because we had to accommodate the requirements of Che. But with the evolution of the task system in code (and Theia), some of those tradeoffs should be revisited.

I know this all sounds awfully vague, but IMO it's an area of technical debt, even though I have no exact plan on how to fix it.

[tsmaeder]

For example: #8399