Allow prefixed names in shape maps without white spaces

[labra]

Although the shape map spec doesn't talk about prefixed names, they are much more useful than long IRIs. Instead of

<http://example.org/alice>@<http://shapes.org/User>

it is more readable to use:

:alice@s:User

At this moment, this feature is supported by the parser, but there is a conflict between the @ employed as a separator between node selectors and shapes, and the @ employed in language tagged literals. The current parser handles it right if there is a white space as:

:alice @ s:User

It would be better to handle it even without the white space, but the ANTLR grammar must probably be changed.