ast: add printing of Variable Expr

examples/hello.v

@@ -9,5 +9,5 @@ fn main(a int) int {
 	1 + 2 + 3 + 4
 	1 + 1 * 1
 	3 / (51 + 2)
-	println(2 * 1956)
+	println(2 * 1956 + a)
 }

src/ast.zig

@@ -199,7 +199,9 @@ pub fn printExpr(expr: *Expr) void {
             }
         },
 
-        else => std.debug.warn("EXPR_UNKNOWN"),
+        .Variable => |token| std.debug.warn("{}", token.lexeme),
+
+        else => std.debug.warn("UnknownExpr-{}", @tagName(expr.*)),
     }
 }