feat: new web middleware

[wolfy1339]

Resolves #623

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Before the change?

• Users of platforms that don't have the unref() function property on setTimeout would get errors
• No web standard middleware was exported

After the change?

• Adds a new web standards middleware
• Bumps the required version of @octokit/webhooks to v18.7.0

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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[akku1139]

It seems to be working fine!
I had some issues with the private key format when trying to get it to work on Cloudflare Workers without nodejs_compat, so maybe it should be added that to the README.

https://github.com/octokit/auth-app.js#standalone-usage
https://github.com/gr2m/universal-github-app-jwt#private-key-formats
https://github.com/gr2m/universal-github-app-jwt/blob/715d7b5f5327e4d49afe99ff9d4aaed3d36a33b0/lib/get-token.js#L23-L27