TinyG with ShuttleXpress: clear queue when zone=0

[Sil3ntLight]

(Unsure if applies to firmwares other than TinyG / g2core, so only implemented for tinyg/g2core)

When jogging with shuttleXpress (any OS, browser or desktop, etc), it has a delay of 1-2 seconds when returning the wheel to center. This makes overshooting and crashing difficult to avoid.

TinyG and g2core require a feedhold command combined with a queue flush when done sending continuous jogging commands, as per:
https://github.com/synthetos/g2/wiki/Feedhold,-Resume,-and-Other-Simple-Commands#jogging-using-feedhold-and-queue-flush

This applies the same to TinyG and g2core both.
Note that the ! and % should not have a space between, otherwise it will only feedhold (this is intentional in g2core, as some gcode generators use % as a comment delimiter)

[TobyGWilliams]

This PR imports yarn but doesn't make use of it, suggest this is addressed.