feat: loop line cursor

[microbit-grace]

Fixes #401
Possible mitigation 1 in the issue

@rachel-fenichel Please feel free to not take this change and implement it differently. Not sure if there is a better way of implementing this.

[rachel-fenichel]

Please also add a loop param to getPrevNode

[microbit-matt-hillsdon]

FYI Grace won't be around until tomorrow so please feel free to take this and run with it.

We may need should pass loop to the recursive calls too.

[microbit-matt-hillsdon]

This has given us looping during constrained move up (e.g. of draw circle in draw in the default demo). But if I move down then it breaks:

b.getSourceBlock is not a function
TypeError: b.getSourceBlock is not a function
    at ConnectionChecker$$module$build$src$core$connection_checker.doSafetyChecks (webpack-internal:///./node_modules/blockly/blockly_compressed.js:1233:272)
    at ConnectionChecker$$module$build$src$core$connection_checker.canConnectWithReason (webpack-internal:///./node_modules/blockly/blockly_compressed.js:1230:658)
    at ConnectionChecker$$module$build$src$core$connection_checker.canConnect (webpack-internal:///./node_modules/blockly/blockly_compressed.js:1230:526)
    at eval (webpack-internal:///./src/keyboard_drag_strategy.ts:88:39)
    at Array.forEach (<anonymous>)
    at KeyboardDragStrategy.getConstrainedConnectionCandidate (webpack-internal:///./src/keyboard_drag_strategy.ts:86:24)
    at KeyboardDragStrategy.getConnectionCandidate (webpack-internal:///./src/keyboard_drag_strategy.ts:111:25)
    at KeyboardDragStrategy.updateConnectionPreview (webpack-internal:///./node_modules/blockly/blockly_compressed.js:914:495)
    at KeyboardDragStrategy.drag (webpack-internal:///./node_modules/blockly/blockly_compressed.js:914:307)
    at KeyboardDragStrategy.drag (webpack-internal:///./src/keyboard_drag_strategy.ts:36:15)

[microbit-matt-hillsdon]

This code has now moved into core so I think someone else will need to take #401 and implement looping in core's version. Suggest closing.

[rachel-fenichel]

Agreed. I'm implementing it in core.