feat: Add composite words to cloze + more lessons

[toshify]

Because our users should get challenged to improve further

this commit will:

• add lessons 13-16 and 18-19 with audio and some pictures
• add composites formed from multiple words to cloze exercises, allowing the user to choose short phrases as anwsers to fill a blank
• fix a minor inconsistency in Lesson 11, where the colons were not Chinese colons (: vs ：)
• increase robustness of LessonValidation.test.ts

Certification

• [X] I certify that
• I have read the contributing guidelines
• I license these contributions to the public under Seedling's LICENSE and have the rights to do so.

Signed-off-by: toshify 4579559+toshify@users.noreply.github.com