[Enhancement] Theme color for member variables

[Grimelios]

Custom themes can use the Identifier key to highlight identifiers, but there's currently no way to visually distinguish between member variables and local variables. As an example, here's a screenshot from Visual Studio where solver is a member variable of the surrounding class, while sourceSet and sourceSetIndex are local variables within the function.

With this distinction, I often use the this. prefix liberally, which does clarify a variable's origin at a glance, but also (arguably) clutters the code.