cleanup: replace C for loops with range

vlib/builtin/js/array.v

@@ -109,7 +109,7 @@ pub fn (a array) free() {
 pub fn (a []string) str() string {
 	mut sb := strings.new_builder(a.len * 3)
 	sb.write('[')
-	for i := 0; i < a.len; i++ {
+	for i in 0..a.len {
 		val := a[i]
 		sb.write('"')
 		sb.write(val)

vlib/builtin/js/int.v

@@ -83,7 +83,7 @@ pub fn (c byte) is_capital() bool {
 
 pub fn (b []byte) clone() []byte {
 	mut res := [byte(0)].repeat(b.len)
-	for i := 0; i < b.len; i++ {
+	for i in 0..b.len {
 		res[i] = b[i]
 	}
 	return res