feat: added solutions to lc problem: No.0020

Created file solution.cs for C# solution to lc problem: No.0020

Author: dev-mauli

solution/0000-0099/0020.Valid Parentheses/solutions.cs

@@ -0,0 +1,16 @@
+public class Solution {
+    public bool IsValid(string s) {
+        Stack<char> ch = new Stack<char>();
+            foreach (var item in s.ToCharArray())
+                if (item == '(')
+                    ch.Push(')');
+                else if (item == '[')
+                    ch.Push(']');
+                else if (item == '{')
+                    ch.Push('}');
+                else if (ch.Count == 0 || ch.Pop() != item)
+                    return false;
+
+            return ch.Count == 0;
+    }
+}