Move support for Beaver Builder into core

[brianhogg]

Is your feature request related to a problem? Please describe.
Users may not trust installing an experimental LifterLabs plugin to get Beaver Builder support.

Describe the solution you'd like

• Move the support for Beaver Builder into core.
• In core, check if the Lab is installed and enabled and if so, do not also load the core lab
• By default, remove the default Course content at the bottom of a Course post (similar to Elementor)
• If possible/quick, add in the default BB modules to a new Course post when you start editing it in Beaver Builder.
• Ensure if labs + core with BB moved over is installed, BB still works as expected.

Describe alternatives you've considered
N/A