Quiz building improvements for Moodle 4.x

There are several changes planned for Moodle 4.1 to improve the experience of building quizzes, both new features, and usability improvements. I have created this Epic to group them.

The changes are also summarised and discussed in this forum thread: https://moodle.org/mod/forum/discuss.php?d=434031.

Overview of the changes:

A. MDL-74608 (force language for an activity) was done in Moodle 4.1.

B. MDL-74609 (customise question number) just missed 4.1, and is now up for integration into Moodle 4.2.

C. So, the remaining big feature is MDL-74610 (let a quiz have multiple grade items) remains. I want to take this change as an opporunity to clean up a lot of techincal debt in the quiz code generally, and more-so in the code related to mod/quiz/edit.php, so there are a sequence of changes here, that need to be done in order, because they all affect the same code, so there will be conflicts if they are done in parallel. See also

1. MDL-74923 move all PHP classes into the classes folder - part 1
2. MDL-76614 move all PHP classes into the classes folder - part 2
3. MDL-76642 improve the structure class, by adding a slot class
4. MDL-76643 Change mod/quiz/edit.php rendering to use templates
5. MDL-76644 replace quiz-building YUI modules with AMD
6. MDL-76646 move library functions that belong there into the structure class
7. MDL-76647 replace edit_rest.php with web services and update the JS to use them
8. MDL-74610 Let a quiz have multiple grade items, finally!

(Order for 3/4/5/6 could be changed.)