Update to Async `Browser`

[inancgumus]

What?

Update relevant Browser methods from sync to async.

Checklist

• [x] I have used a meaningful title for the PR.
• [x] I have described the changes I've made in the "What?" section above.
• [x] I have performed a self-review of my changes.
• [x] I have run the npm start command locally and verified that the changes look good.

• [x] I have made my changes in the docs/sources/next folder of the documentation.
• [ ] I have reflected my changes in the docs/sources/v{most_recent_release} folder of the documentation.
• [ ] I have reflected my changes in the relevant folders of the two previous k6 versions of the documentation (if still applicable to previous versions).

Related PR(s)/Issue(s)

grafana/xk6-browser#1294

[inancgumus]

@heitortsergent We're converting all relevant k6-browser methods to async. And, we'll be merging our changes into main-browser-async before finally merging them into main. We do this because it's not guaranteed that we'll make all the sync-to-async conversion until the next release.

Would you like to review the PRs individually, or the final one just before we merge all of them into main?