Switch to handle Core/5.6 compat now that Application Passwords is merged.

[georgestephanis]

• Shift the classic plugin files into a subdirectory.
• Make a new subdir for the newer implementation assets.
• Have the main plugin loader file switch based on the core class being defined.

Goal for this PR is to still work as-previously for older versions of WP, but also seamlessly transition to a playground for future enhancement for for newer versions.

Still a WIP for now.

Things that the new compat mode for 5.6 should probably handle:

• Redirect requests from the legacy auth_app admin page to the new core implementation.
• Either reimplement the legacy endpoints, or provide a deprecation notice and details on the new ones?
• Be a testing ground for new admin ui overhauls for the user profile screen or the like.

Fixes #124

[georgestephanis]

Tentative page looks something like this -- still open to just calling wp_safe_redirect() to pass the user along silently though. Not sure if this sort of interstitial would be better at prompting folks to update their urls in their apps.