Generate 'isWatching' could be true if task has 'watch' in name

[isidorn]

Related to #4019

When generating a tasks.json we always set the isWatching to false, however we could use a simple heuristic to set this to true if a task name has 'watch' in the name. IMHO this heuristic could work nicely in practice.

To repro, just generate tasks.json for vscode repo

[dbaeumer]

@isidorn nice idea, but not easy to realize. The problem is that when in watching mode you need special regular expression to determine in the output when the task is active and when inactive. Otherwise the spinner will not stop :-).

[dbaeumer]

For task 2.0.0 I implemented that isBackground will be true if the associated matcher has a background property. This is a stronger indication than a name.