What does this PR do?

Implement A/B experiment for sticky navigation.

Explanation

1. On initial load, we pull in the feature flag from PostHog in layout.server.ts, assigning a distinctId based off of available request headers. This is persisted in memory. The data from the feature flag for a given user is returned.
2. The distinctId is passed into a child layout.ts via the data, and in the browser we initialize the PostHog client that tracks events like button clicks etc. with the distinctId that is created in the server layout. This is also explicitly persisted in memory. All the data is then passed to a root page.ts.
3. Via $page.data, we access the isStickyNav variable in Main.svelte which is the parent wrapper for pages with the default header.

API

Inside of src/lib/experiments.ts there's a new function called getFeatureFlag; when we add new experiments via the PostHog UI, we should add the key, as well as the variant names to the experiments object, which will ensure that we always have typesafety, and parity when we use our feature flags.

Test Plan

Manual

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