Migrate Browser APIs to async

[ankur22]

What

Migrate the following APIs according to the How section in #428:

Class	Method	Current k6 Return	PW return
Browser	closeContext	void	not found in Playwright
Browser	context	BrowserContext	not found in Playwright
Browser	newContext	BrowserContext	Promise
Browser	newPage	Page	Promise

For details on the Why and How, check the overarching issue in #428.

Tasks

Beta Give feedback

1. Async browser.newContext #1310
   async +1
   
2. Async browser.closeContext #1311
   async +1
   
3. Async browser.newPage #1312
   async +1
   
4. Fix page.goto tests for async #1316
   async bug tests +3
   
5. Update to Async Browser k6-docs#1588
   Area: browser +1
   
6. Update Browser/BrowserContext API:s to return Promise k6-DefinitelyTyped#44
   +0
   
7. Add release notes