arg method
Create a DeclArg node corresponding to the id.
If optional is true, then any generated HTML documentation
will decorate this parameter with '[]' pairs. This named argument
does nothing to the semantics or the lua runtime.
Implementation
LuaFuncBuilder arg(String id, {bool optional = false}) =>
this..args.add(DeclArg(Token.synthesized(id), isOptional: optional));