Add QEnumIndexedArray

Write a container adapter that converts indices from/to enums, to get an enum-indexed container.

There's a QEnumIndexedArray in https://codereview.qt-project.org/c/qt/qtbase/+/425118, but it's limited to std::array. Would probably also want one for std::vector / QList / QVLA, so more like std::queue than https://codereview.qt-project.org/c/qt/qtbase/+/425118's current implementation.

Acceptance criteria:

• Write a QEnumIndexed container wrapper that can wrap at least
  • std::array
  • std::vector
  • QList (NB: qizetype size_type)
  • QVLA
• Add template aliases
  • QEnumIndexedArray (std:::array)
  • QEnumIndexedVector (std::vector)
  • QEnumIndexedList (QList)
  • QEnumIndexedVariableLengthArrar (QVLA)
• find pertinent code and add two in-tree users
  • there's at least https://codereview.qt-project.org/c/qt/qtpositioning/+/425095
  • IIRC, there's a Qt::Cursor-indexed array in one of the wayland classes, too
  • ...