Make systems standalone functions

[Shatur]

I used to put systems under plugin impls to clarify that the system belongs to a plugin.
But looks like in Bevy using standalone systems is more common. I think it's especially makes sense now with observers. Let's follow this pattern.

[codecov]

Codecov Report

Attention: Patch coverage is 97.65101% with 7 lines in your changes missing coverage. Please review.

Project coverage is 89.78%. Comparing base (c2372b7) to head (eccd340).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/server.rs	93.68%	6 Missing ⚠️
src/parent_sync.rs	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #403      +/-   ##
==========================================
- Coverage   89.81%   89.78%   -0.03%     
==========================================
  Files          51       51              
  Lines        2778     2771       -7     
==========================================
- Hits         2495     2488       -7     
  Misses        283      283              

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