Generate Spin build task as the default build task

[itowlson]

{
	"type": "spin",
	"command": "build",
	"options": [],
	"problemMatcher": [],
	"label": "spin: build",
	"group": {
		"kind": "build",
		"isDefault": true
	}
}

[itowlson]

Hmm, I think I can add the group but I don't know if I can make it the default:

[itowlson]

It works if the user runs the Configure Default Build Task command, but then the prompt shows both build and up even though only build is defined as TaskGroup.Build...

[itowlson]

This seems like it could be something we could preset in the templates, so they came with default build and test tasks that hooked into the extension...