feat: added in expression in if tag (#143)

Sources/Expression.swift

@@ -85,6 +85,37 @@ final class NotExpression: Expression, PrefixOperator, CustomStringConvertible {
   }
 }
 
+final class InExpression: Expression, InfixOperator, CustomStringConvertible {
+  let lhs: Expression
+  let rhs: Expression
+  
+  init(lhs: Expression, rhs: Expression) {
+    self.lhs = lhs
+    self.rhs = rhs
+  }
+  
+  var description: String {
+    return "(\(lhs) in \(rhs))"
+  }
+  
+  func evaluate(context: Context) throws -> Bool {
+    if let lhs = lhs as? VariableExpression, let rhs = rhs as? VariableExpression {
+      let lhsValue = try lhs.variable.resolve(context)
+      let rhsValue = try rhs.variable.resolve(context)
+      
+      if let lhs = lhsValue as? AnyHashable, let rhs = rhsValue as? [AnyHashable] {
+        return rhs.contains(lhs)
+      } else if let lhs = lhsValue as? String, let rhs = rhsValue as? String {
+        return rhs.contains(lhs)
+      } else if lhsValue == nil && rhsValue == nil {
+        return true
+      }
+    }
+    
+    return false
+  }
+  
+}
 
 final class OrExpression: Expression, InfixOperator, CustomStringConvertible {
   let lhs: Expression