Refactoring - Simplify and Modernise Code

[hgeorgsch]

The code should be reviewed with the following goals

1. Harmonise the code with the core quiz, using similar patterns where possible.
2. Benefit fully from the Moodle 4 qbank API.
3. Avoid redundant code.

[sebastsg]

Mentioning #231 here due to new qbank api usage, but it also modernizes a lot of code with return and param types, shortened array syntax, and removing/cleaning weird code - f.ex checking if new doesn't return false/null...)