Browser: add route-related functions

[AgnesToulet]

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• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change.
• Follow the advice from the readme.
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• Run pnpm test <package to test>.

Select one of these and delete the others:

If adding a new definition:

• Provide a URL to documentation or source code which provides context for the suggested changes:
  Browser: add page.route k6#4953
  Browser: add route.continue k6#4971
  Browser: add route.fulfill k6#4961