[wp-now] Deprecate wp-now, display a warning in the CLI

[adamziel]

Officially recommends using Playground CLI instead of wp-now by:

• Displaying a visible warning when the wp-now command runs
• Updating wp-now README

Rationale

wp-now is not actively developed and has many unresolved bugs.
Meanwhile, Playground CLI lives in the core Playground
repo and is up-to-date with core by definition. It has less issues
and receives more patches.

Why now?

Up until now, Playground CLI was missing an automatic context recognition
feature that wp-now has. It identifies whether the script is ran in a site,
theme, or a plugin directory and mounts it to a correct location in
Playground's virtual filesystem. Today, Playground CLI is about to
get that same feature which makes this deprecation possible

Remaining work

• Wait until the latest Playground CLI changes are merged
• Update Playground CLI's README with npx-based examples for the
  package consumers. The current README is focused on developers. Cover
  the same bases as wp-now README to make the transition smooth. Explain
  the different thinking models – wp-now has persistent sites implicitly
  assigned to disk paths, Playground CLI has a more manual approach to
  managing site persistence. wp-now implicitly mounts the current CWD,
  Playground CLI has explicit flags and an --autoMount option.
• Tag a new Playground CLI release to publish that README on npm
• Merge this PR
• Tag a new wp-now release