feat: implement `waitForInitialization` for browser sdk 4.x#1028

joker23 merged 13 commits intomainlaunchdarkly/js-core:mainfrom

skz/sdk-1360/browser-4.x-waitforinitlaunchdarkly/js-core:skz/sdk-1360/browser-4.x-waitforinitCopy head branch name to clipboard