workspaces: Support member grouping

[shadows-withal]

From #536. A workspace member could accept a group key, for example.

[jamesmunns]

If you are able to preserve ordering, it might be easier to take an array of arrays for members, like:

"members": [
    [
        { "slug": "a", "path": "./a" },
        { "slug": "b", "path": "./b" },
    ],
    [
        { "slug": "c", "path": "./c" },
        { "slug": "d", "path": "./d" },
    ]
]

or as named groups:

"groups": [
    {
        "groupname": "one",
        "members": [
            { "slug": "a", "path": "./a" },
            { "slug": "b", "path": "./b" },
        ]
    }
]

That way you don't also have to define group ordering somewhere else.