Browser: Register sync and async JS APIs (k6-core side)

[inancgumus]

What

Expose two JS import paths for the browser module:

• k6/browser -> The new async API
• k6/experimental/browser -> The old sync API

Why

The browser team has been working on migrating the existing API to Async.

With v0.52.0's release date two weeks away, we're confident that the new Async API can be delivered in one shot. Since these changes are the last piece of the API design and we don't anticipate any breaking changes in the near future, it's worth discussing the graduation of the experimental Browser API to a stable module.

Here are a few reasons that brought us to this idea:

• With the async API, we're aligned with Playwright, and we don't expect any further breaking changes to bridge this gap.
• Releasing the new API may make future changes in the import path more challenging.
• Releasing the async version of the API under a separate import path, k6/browser, while keeping k6/experimental/browser for a while during the deprecation window is a non-breaking change for existing user scripts.