checks: Sync engines across all package.json files

[anomiex]

Changes proposed in this Pull Request:

The engines key in package.json specifies acceptable versions of node
and yarn, and should be the same in all package.json files in the
monorepo. Add a check to ensure that's the case.

Jetpack product discussion

Came up in passing in review of #19705.

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Is CI happy?
• Try changing or removing the engines value in a package.json, see that it reports errors properly.

[test-case-reminder]

Here are some suggested test cases for this PR.

Connection

• In-place connection with free plan
• In-place connection with paid plan
• In-place connection with product purchase
• Classic connection. Use Safari, or set a constant JETPACK_SHOULD_NOT_USE_CONNECTION_IFRAME to true
• Disconnect/reconnect connection
• Secondary user connection
• Connection on multisite

Verify that the changes are compatible with the plugins that use the connection package.

• WooCommerce Payments
• Jetpack Boost
• Previous versions of Jetpack

If you think that suggestions should be improved please edit the configuration file here. You can also modify/add test-suites to be used in the configuration file.

[github-actions]

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[jeherve]

This looks good, but it seems it will require a rebase now.