docs: Update to the use of arrow functions in line with the style guide

[flyme2bluemoon]

Description of Change

Updating the tutorial docs so that the code examples use the new Javascript arrow function syntax rather than the old function () {...} syntax to be in line with Electron coding style guide.

Checklist

• relevant documentation is changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: none

[welcome]

💖 Thanks for opening this pull request! 💖

We use semantic commit messages to streamline the release process. Before your pull request can be merged, you should update your pull request title to start with a semantic prefix.

Examples of commit messages with semantic prefixes:

• fix: don't overwrite prevent_default if default wasn't prevented
• feat: add app.isPackaged() method
• docs: app.isDefaultProtocolClient is now available on Linux

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• Follow the JavaScript, C++, and Python coding style.
• Run npm run lint locally to catch formatting errors earlier.
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• Include tests when adding/changing behavior.
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[ckerr]

Linter issues:

   tutorial/automated-testing-with-a-custom-driver.md
         88:25:    'onMessage' was used before it was defined.

   tutorial/quick-start.md
         386:27:   Parsing error: Unexpected token )

[flyme2bluemoon]

Hey @ckerr,

I have fixed the unmatched bracket in tutorial/quick-start.md. However, the "used before define" issue was already in the docs before this PR.

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉

[release-clerk]

No Release Notes

[trop]

@malept has manually backported this PR to "15-x-y", please check out #30881

[malept]

@malept has manually backported this PR to "15-x-y", please check out #30881

This is a partial backport...