Remove reduce in Playground flags string construction

Author: chenglou

src/common/WarningFlagDescription.mjs

@@ -483,10 +483,10 @@ function merge(base, other) {
 }
 
 function tokensToString(tokens) {
-  return Belt_Array.reduce(tokens, "", (function (acc, token) {
+  return tokens.map(function (token) {
                 var modifier = token.enabled ? "+" : "-";
-                return acc + (modifier + token.flag);
-              }));
+                return modifier + token.flag;
+              }).join("");
 }
 
 var Parser = {

src/common/WarningFlagDescription.res

@@ -280,10 +280,12 @@ module Parser = {
   }
 
   // Creates a compiler compatible warning flag string
-  let tokensToString = (tokens: array<token>): string =>
-    Belt.Array.reduce(tokens, "", (acc, token) => {
+  let tokensToString = tokens => {
+    tokens
+    ->Js.Array2.map(token => {
       let modifier = token.enabled ? "+" : "-"
-
-      acc ++ (modifier ++ token.flag)
+      modifier ++ token.flag
     })
+    ->Js.Array2.joinWith("")
+  }
 }