Add [attribute: value] shorthand

[neunenak]

#2086

Adds a shorthand syntax [attribute: value] for attributes with a single argument, that is equivalent to [attribute(value)].

[casey]

I like the [attribute: value] shorthand, but I don't think we should parse multiple attribute values until we actually have an attribute that takes multiple values, so Attribute:new should be changed back to take an Option<StringLiteral>

[casey]

One more thing: Can you add a separate [attribute: value] test instead of changing the existing group test? I think we can have a single test which tests the short syntax, and leave the other tests as they are.

[casey]

LGTM!