fix: show correct preview during unconstrained movement near a connection

[rachel-fenichel]

Proposed fix for #439

@microbit-robert @microbit-matt-hillsdon I'd appreciate your opinion on this.

Two parts to the change:

Initial offset

In mover.ts, call info.updateTotalDelta immediately after onDragStart.
If the drag immediately shows a preview, this will update the block's location accordingly.
If a preview is shown, the block is moved down and to the right of the target connection.

Rationale

Without this change, the block was wrong about its position until the first constrained movement occurred. I think this change is necessary no matter what.

Offset during drag

In keyboard_drag_strategy.ts, only force the block's position offset if the most recent movement was constrained.

If the movement was unconstrained, the insertion marker still shows or hides as appropriate, but the dragging block can translate relative to the insertion marker. All of these become legal:

Moving away more causes the insertion marker to disappear:

And here's an example as a gif. All movements in this gif are unconstrained.

Rationale

In discussion with Robert and Matt I suggested making the drag step larger during keyboard drags, or making it larger only when showing a preview and trying to do an uncosntrained movement. But as the drag step size increases, so does the chance of missing a connection entirely. This is especially acute when working with value inputs, where the connections may be very close to each other in a row.

While investigating, I realized that part of the issue is that the user has no feedback: there is no visible change on the first unconstrained movement away from a connection (or, in MakeCode, the first four). In mouse mode the feedback is that the block moves along with the mouse pointer. The insertion marker sticks at the new position, but the dragging block moves around. In fact, this is half of the reason for using a separate insertion marker as a preview.

This change mimics mouse dragging by moving the dragging block around while still showing the preview. It may take multiple keypresses to move away from a connection, but users get feedback that a movement is happening.

[microbit-matt-hillsdon]

I think this works really well, thank you.