Docs: Migration guide for k6 browser v1.0.0 API

[ka3de]

After the recent changes made to k6 browser API, including the scenario browser options (grafana/xk6-browser#876), the abstraction of the browser lifecycle (grafana/xk6-browser#910), and the enforcing of a single browserContext per iteration (grafana/xk6-browser#929), current k6 browser scripts API have a few considerable differences from previous versions.

It would probably be helpful for current users to have a migration guide on how to adapt their "old" k6 browser test scripts to the new version.

Note: Final released version might be v.1.0.0 instead of v0.11.0.

[inancgumus]

Here's the current outline:

• Executor and scenario options.
• Changes in configuring the browser: scenario options.
• Execution model changes: One browser one browser context.
• A single scenario can only run contexts in a sequence, not concurrently.
  • Retrieving the browserContext if newPage is used, and how we only retrieve the one live browser context.
  • browser.close is no longer needed.
  • Explain how we now handle the browser lifecycle.
  • Imports have changed: There is no more chromium browser type etc, but browser.
• Environment variable changes.
  • No more K6_BROWSER_ENABLED flag.
• Some of the cmd flags don't work, e.g.: -u, -v and -d.