Option to allow strings for "var-name-mixedcase"

We frequently want to define strings like so, and they act as immutables: ```solidity string internal RESULTS_DIR; contructor() { RESULTS_DIR = "foo"; } ``` Solhint throws this linting warning: ``` 23:5 warning Variable name must be in mixedCase var-name-mixedcase ``` So end up having to disable `var-name-mixedcase`, although we still want to use it for the other variable types. It would be helpful to be able to exempt strings from this rule.