EXIT sequence step

A sequence command to end (stop/abort) the sequence at the current point.

Some possible options or extensions:
* exit recursively: Also exit all parent sequences. Would allow use in an IF statement.
* exit with or without error: Similar to `0` or `1` exit codes or error raise. Sequences errors cause E-STOPs.
* exit with message: This step might often be preceded by a SEND MESSAGE step.
* pause only: Allow resuming of sequence via user input.

Further info and background: https://forum.farmbot.org/t/an-e-stop-command-for-sequences/2766/6