Allow the workspace to be panned when a block is dragged near the edge

[maribethb]

If a block is being dragged near/to the edge of the workspace, the workspace could pan/scroll in that direction.
This might be better as a plugin, but either way there may be changes needed in core to support that.

Additional context

code.org has implemented this in their fork. also, Monica notes "In our WorkspaceSearch plugin we have an implementation called scrollToVisible_ that scrolls the workspace to bring the specified block into view that we could port to core and reuse for this purpose."

This was mentioned elsewhere eg #2205 (comment)

[moniika]

Is this part of core changes needed to enable google/blockly-samples#372?

[maribethb]

Yeah, there are some changes needed in core to support the plugin so this issue is for those changes.