feat: let mut 改为 var 关键字声明可变变量

test/programs/06_mut_while.l

@@ -1,5 +1,5 @@
 fn main() -> i64 {
-    let mut i: i64 = 0;
+    var i: i64 = 0;
     while i < 5 {
         print_i64(i);
         i = i + 1;

test/programs/09_compound_assign.l

@@ -1,5 +1,5 @@
 fn main() -> i64 {
-    let mut x: i64 = 10;
+    var x: i64 = 10;
     x += 5;
     print_i64(x);   // 15
     x -= 3;

test/test_sema.c

@@ -55,7 +55,7 @@ void test_let_mut_assign_ok() {
     Arena a = arena_create(1);
     size_t tc; ErrorInfo lex_err = {0};
     Token* toks = lex(&a,
-        "fn main() { let mut x: i64 = 0; x = 42; print_i64(x); return; }",
+        "fn main() { var x: i64 = 0; x = 42; print_i64(x); return; }",
         "test", &tc, &lex_err);
     ASSERT(toks != NULL);
     ErrorInfo parse_err = {0};