feat: add ethernaut lvl 19

src/EthernautCTF/AlienCode.sol

@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.5.0;
+
+import '@openzeppelin-05/ownership/Ownable.sol';
+
+contract AlienCodex is Ownable {
+  bool public contact;
+  bytes32[] public codex;
+
+  modifier contacted() {
+    assert(contact);
+    _;
+  }
+
+  function makeContact() public {
+    contact = true;
+  }
+
+  function record(bytes32 _content) public contacted {
+    codex.push(_content);
+  }
+
+  function retract() public contacted {
+    codex.length--;
+  }
+
+  function revise(uint256 i, bytes32 _content) public contacted {
+    codex[i] = _content;
+  }
+}