Split expressions into separate files

src/Parser/Expression/ExpressionVariable.cpp

@@ -0,0 +1,12 @@
+#include "ExpressionVariable.h"
+
+ExpressionVariable::ExpressionVariable(string name):
+Expression(ExpressionKind::VAR, ValueType::NONE), name(name) { }
+
+string ExpressionVariable::getName() {
+    return name;
+}
+
+string ExpressionVariable::toString(int indent) {
+    return "VAR(" + name + ")";
+}