CLI: Determine if cwd is within _any_ JP monorepo

[kraftbj]

Steps to reproduce the issue

1. Setup the Jetpack monorepo per the usual instructions, including the cli-setup step.
2. Checkout a second copy of the monorepo somewhere else.
3. From the second copy, run jetpack build plugins/jetpack

What I expected

The second copy has Jetpack built.

What happened instead

The first copy built Jetpack.

See discussion at p1626875884194600/1626874210.189400-slack-CBG1CP4EN

What's happening is in the Jetpack bin, we're setting the cwd to the monorepo root of the bin. We do this so you can use the CLI from any point in the monorepo without worrying about needing to be at the monorepo root.

We should detect if we're within a Jetpack monorepo and, if we are, use that monorepo's root. If we're not within one, I suppose using the bin's root is sufficient (or should we error? I could see docker commands being used outside of the monorepo?)

[anomiex]

Note you'll probably also need to do something about these too:

tools/cli/commands/changelog.js:		const projPath = path.join( __dirname, '../../..', 'projects', proj, 'changelog', fileName );
tools/cli/commands/generate.js:	const projDir = path.join( __dirname, '../../..', 'projects/', type, answers.name );

Or else if you detect that you're in a different Jetpack monorepo, exec that monorepo's copy of the Jetpack CLI instead of continuing to run the current one.

[jeherve]

At this point I think we can close this one, so we're now focusing on things like #40934.