Added command M181.

[lromor]

• Parser shall route to unprocessed commands.
• GCodeMachineControl shall handle the command and set the value to max if no S pair is provided. Added single test that only checks the motor ops queue shall be emptied before changing the planner size.
• Planner respect the lookahead value and always push older commands to maintain planning queue size. No tests added.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +1.72 :tada:

Comparison is base (4d55223) 45.93% compared to head (302fd25) 47.66%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #66 +/- ## ========================================== + Coverage 45.93% 47.66% +1.72% ========================================== Files 37 37 Lines 4245 4284 +39 ========================================== + Hits 1950 2042 +92 + Misses 2295 2242 -53 ``` | [Impacted Files](https://codecov.io/gh/hzeller/beagleg/pull/66?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Henner+Zeller) | Coverage Δ | | |---|---|---| | [src/gcode-machine-control.cc](https://codecov.io/gh/hzeller/beagleg/pull/66?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Henner+Zeller#diff-c3JjL2djb2RlLW1hY2hpbmUtY29udHJvbC5jYw==) | `30.41% <100.00%> (+10.30%)` | :arrow_up: | | [src/planner.cc](https://codecov.io/gh/hzeller/beagleg/pull/66?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Henner+Zeller#diff-c3JjL3BsYW5uZXIuY2M=) | `83.74% <100.00%> (+0.79%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://codecov.io/gh/hzeller/beagleg/pull/66/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Henner+Zeller) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Henner+Zeller). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Henner+Zeller)

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[lromor]

@hzeller , I've added a planner test to check the lookahead resize behavior. Let me know what you think!

[hzeller]

In the gcode_finished() callback the GCodeMachineControl receives: might be useful to also reset the planner buffer to normal.