For representing Identifiers that have an implicit leading $.
With this done, maybe go one step further?
* Rename VarLikeIdentifier -> VarIdentifier / VarName
* Use VarIdentifier / VarName also as an inner node in Variable.
Not sure if this adds any real value.
In this mode non-namespaced names that are currently represented
using strings will be represented using Identifier nodes instead.
Identifier nodes have a string $name subnode and coerce to string.
This allows preserving attributes and in particular location
information on identifiers.
