Add arithmetic expressions (SET /a)

Please add [`SET /a`](https://ss64.com/nt/set.html#expressions) command line switch with mathematical expressions, using e.g. [TinyExpr](https://github.com/codeplea/tinyexpr), [expr](https://github.com/zserge/expr) or [expression_parser](https://github.com/jamesgregson/expression_parser).