Support VS Code task configuration format

[marcdumais-work]

Our current tasks.json format is somwwhat similar but not as rich. Supporting the VS Code format would open the door to being able to use existing tasks from that IDE. Even a partial support could be worthwhile.

Our current format:
https://github.com/theia-ide/theia/tree/master/packages/task

VS Code format:
https://code.visualstudio.com/docs/editor/tasks-appendix#_schema-for-tasksjson

to consider: Explore future direction of the task runner system
microsoft/vscode#15179

[akosyakov]

Just stumbled on it, trying to run first typescript task from VS Code docs: https://code.visualstudio.com/docs/editor/tasks

Also i could not find a command to configure tasks and had to create .theia/tasks.json file myself. End users don't know it.

[tsmaeder]

I think we do support the format now.