Fix ts-node version issue with webext sample project

[rosahbruno]

Description

I discovered that the webext tests were broken with newer versions of node. This is the same issue that we saw with the main glean package tests. I did not realize at first that this sample project also had its own tests and the same issues as the glean tests. The same fix that we did for glean proper can be applied here.

Notes on the other sample projects tests

• web - This project does not have tests, so there is nothing to fix.
• node - This project uses mocha directly for testing, it does not do any of the experimental ESM stuff like glean & webext
• qt - The tests for this project has issues, but they are not related to the ts-node versions. I am logging a bug for those to be worked on separately.

Testing

1. Run npm run test from inside of the samples/browser/webext folder with a node version greater than 10.5.x.
2. Run the sample project and confirm you can see pings in the debug viewer.

Pull Request checklist

• Quality: Make sure this PR builds and runs cleanly.
  • Inside the glean/ folder, run:
    • npm run test Runs all tests
    • npm run lint Runs all linters
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry to CHANGELOG.md or an explanation of why it does not need one
• Documentation: This PR includes documentation changes, an explanation of why it does not need that or a follow-up bug has been filed to do that work
  • Documentation should be added to The Glean Book when making changes to Glean's user facing API
    • When changes to the Glean Book are necessary, link to the corresponding PR on the mozilla/glean repository
  • Documentation should be added to the Glean.js developer documentation when making internal code changes

[chutten]

LGTM