Add support for extended attributes identifier lists

[tobie]

This adds support for constructs of the form:

[Exposed=(Window,Worker)]

which were kind of supported, but not documented.

The proposed JSON syntax for the above would be:

{
    "name": "Exposed",
    "arguments": null,
    "rhs": {
        "type": "identifier-list",
        "value": [
            "Global",
            "Worker"
        ]
   }
}

This introduces a new Right Hand Side type, the identifier list. I'm happy to change the JSON syntax, though, if so, please LMK.

Note that currently, the above would be rendered as:

{
    "name": "Exposed",
    "arguments": null,
    "rhs": [
        "Global",
        "Worker"
    ]
}

[jyasskin]

Looks fine to me.

[tobie]

Oh, had missed your review here. Thanks!

Just want to make sure @darobin is confortable with the introduction of the identifier-list RHS type.