Add support for inline components

Currently new composite types require a separate .qml file. To faciliate various features we would like to be able to specify new types also within a .qml file. The proposed syntax introduces a new lower-case component keyword.

Suppose we have Button.qml:

import QtQuick 5.15
Item {
    ...
    component StandardBorder : Rectangle {
        property color color;
        ...
    }
    ...
}

Such QML code would make a new type Button.StandardBorder available for use as regular type for instantiation.