The default value for this parameter, if provided.
Note that there are no unpacked dimensions here, even though it is
legal to write things like parameter [3:0] foo [4:0], because we have
not seen a case where the above sort of parameter declaration differs from a
fully explicitly typed parameter such as parameter logic [3:0] foo
[4:0]. It's not entirely clear that this is the right behavior, but at
present the parser will create vl-explicitvalueparams for any
parameters that have unpacked dimensions, as if they had been specified
with a logic type.
