QVarLengthArray: add missing STL-style assign()

See epic for more details.

Acceptance criteria:

• QVLA has an assign() overload set following std::vector's (https://en.cppreference.com/w/cpp/container/vector/assign), except for the C++20 constexpr
• For a QVLA that has sufficient capacity() to hold the result of the operation, these functions do not allocate additional memory.
• All functions are strongly exception safe if value_type copy construction doesn't throw.