Implicit build task should be the debug build task

[ddeclerck]

So that launching debugging when there is no tasks.json file works.

I.e the debug build task in tasks.json should contain:

			"group": {
				"kind": "build",
				"isDefault": true
			}

That should probably be handled in attributes_spec (superbol_tasks.ml).

[nberth]

isDefault does not seem to be modifiable via the API (cf https://vscode-api.js.org/classes/vscode.TaskGroup.html#isDefault). There's probably a better way to have the extension work properly without the tasks.json.