js: fix string.bytes codegen, readline, add tests for strings (#12060)

vlib/strings/strings.js.v

@@ -9,10 +9,8 @@ pub fn repeat(c byte, n int) string {
 }
 
 pub fn repeat_string(s string, n int) string {
-	return ''
-	/*
-	// TODO: uncomment this. It is commented for now, so that `v doc strings` works
-	res := # s.repeat(n)
+	res := ''
+	#res.str = s.str.repeat(n.valueOf())
+
 	return res
-	*/
 }