[fix] improve style resolver performance

Script time in the benchmark was profiled by adding `console.profile` around
the timings for script time. The call to Array.join in the resolve function
stood out. Since the code already iterates over the array it can run slightly
faster by building the cache key in that loop instead.

Close #1213

packages/react-native-web/src/exports/StyleSheet/ReactNativeStyleResolver.js

@@ -84,15 +84,19 @@ export default class ReactNativeStyleResolver {
     // otherwise fallback to resolving
     const flatArray = flattenArray(style);
     let isArrayOfNumbers = true;
+    let cacheKey = '';
     for (let i = 0; i < flatArray.length; i++) {
       const id = flatArray[i];
       if (typeof id !== 'number') {
         isArrayOfNumbers = false;
       } else {
+        if (isArrayOfNumbers) {
+          cacheKey += (id + '-');
+        }
         this._injectRegisteredStyle(id);
       }
     }
-    const key = isArrayOfNumbers ? createCacheKey(flatArray.join('-')) : null;
+    const key = isArrayOfNumbers ? createCacheKey(cacheKey) : null;
     return this._resolveStyleIfNeeded(flatArray, key);
   }