[wip] Replace LegacyHidden with Activity mode

[rickhanlonii]

Based off PR: #32499

There's a comment in the code added in this commit that we should replace LegacyHidden with an Activity mode:

react/packages/react-server/src/ReactFizzServer.js

Lines 2253 to 2255 in 54fdb2f

	// TODO: Delete in LegacyHidden. It's an unstable API only used in the
	// www build. As a migration step, we could add a special prop to Offscreen
	// that simulates the old behavior (no hiding, no change to effects).

This PR is an example of what that could look like, but it also exposes how difficult it is in the current model to add different modes with different behaviors. We could merge this, get on this version, and then iterate and improve.

Or we could just use this as an example to figure out the right model and layering.

Note: this kinda just ignores the view transition stuff for the legacy modes.