Variable declarations seperate from assignment

src/lang/Nub.Lang/Frontend/Lexing/Lexer.cs

@@ -17,6 +17,7 @@ public class Lexer
         ["return"] = Symbol.Return,
         ["new"] = Symbol.New,
         ["struct"] = Symbol.Struct,
+        ["let"] = Symbol.Let,
     };
 
     private static readonly Dictionary<string, Modifier> Modifiers = new()