Add PostHog feature flag example

[leggetter]

PostHog is an open-source product analytics platform that offers a suite of tools, including funnels, heat maps, session recording, feature flags, and more, all in a single platform. This PR adds an example - based on the existing Split and ConfigCat examples - of using PostHog Feature Flags to A/B test a Next.js app using the new Next.js 12 middleware functionality.

I haven't been able to test out the deploy functionality from this example as the URLs for the source won't exist until the PR is merged.

The feature flags are:

• New_Marketing_Page: A/B test
• New_About_Page: A/B test
• New_Product_Page: Multivariate test with values of a, b, or c

A/B example:

Multivariate example:

Feature flag insight analysis:

[vercel]

@leggetter is attempting to deploy a commit to the Vercel Team on Vercel.

A member of the Team first needs to authorize it.

[leggetter]

I've also just added a multivariate example with a Product page to go with the two A/B test page examples.