Introduce Concept of "Execution" for Tasks

[tsmaeder]

Many problems (#8822, https://github.com/eclipse-theia/theia/pull/8756/files#r525950124, etc.) seem to be caused by the fact that we don't have the concept of "Execution" (as in ProcessExecution, ShellExecution) in Theia. Instead, we rely solely on the task type to determine which TaskRunner to use for a particular task. So we end up with two "task type" fields, namely type and taskType. This leads to code that is hard to understand, maintain and test.

Feature Description:

I would introduce the the concept of "execution" into the Task system (in TaskConfiguration, for example) and use that to choose a task runner. The task type would then remain static and we could have a single type field for tasks. The current mechanism of registering a task runner for a particular task type should remain for compatibility reasons at this moment and could be deprecated over time.