Choose which blocks to drag based on modifier key

[rachel-fenichel]

Once RaspberryPiFoundation/blockly#8870 is fixed in core (and released in a beta for use), override shouldHealStack to return false if shift was pressed and true otherwise.

This corresponds to a single-block drag with m (default) and a multi-block drag with M.

As a first pass, this will require passing additional information into the drag strategy through side channels in the event, as I did with the direction of movement.

Later, the information should be passed in directly. I think that we may change the dragging code to accept a keyboard event | pointer event instead of just a pointer event.

[microbit-matt-hillsdon]

We should make a corresponding change to the disconnect operation at the same time for consistency (it was updated to be consistent with move on #542).

[gonfunko]

This was addressed in RaspberryPiFoundation/blockly#9630.