[relax-ng] attributes in compact syntax

Since we allow

<attribute name="glorp"/>

to be equivalent to

<attribute name="glorp"><text/></attribute>

couldn't we, in compact syntax, allow

attribute glorp,

to be the equivalent of

attribute glorp { text }, ?

In other words, if the next token after the attribute's name is a comma and
not a curly brace, the processor writes <text/>.

Mike