Enable _lesson_preview show-in-rest meta settings

[onubrooks]

Fixes #

Changes proposed in this Pull Request

• Adds settings to show lesson_preview post meta in the lessons REST API.

Testing instructions

• The easiest way to test this is in the block editor using the following snippet:
  useSelect(() => { return select('core/editor').getCurrentPostAttribute( 'meta' ); })

• Ensure _lesson_preview key is present in the object returned.

[codecov]

Codecov Report

Merging #5369 (089de58) into trunk (5fe92a8) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##              trunk    #5369      +/-   ##
============================================
- Coverage     40.82%   40.81%   -0.01%     
  Complexity     8639     8639              
============================================
  Files           285      285              
  Lines         29127    29133       +6     
============================================
  Hits          11891    11891              
- Misses        17236    17242       +6     

Impacted Files	Coverage Δ
...t-api/class-sensei-rest-api-lessons-controller.php	9.83% <0.00%> (-1.08%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 37cc8aa...089de58. Read the comment docs.