QList: add missing STL-style assign()

See epic for more details.

Acceptance criteria:

• QList has an assign() overload set following std::vector's (https://en.cppreference.com/w/cpp/container/vector/assign), except the C++20 constexpr, and using QList::parameter_type instead of const T&.
• For a QList that is not shared and has sufficient capacity() to hold the result of the operation, these functions do not allocate additional memory.
• Otherwise, equivalent to *this = QList(~~~args~~~).