CompatHelper: bump compat for PowerModels to 0.19, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the PowerModels package from 0.18 to 0.18, 0.19.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov]

Codecov Report

Merging #63 (f80054c) into develop (0ccb4d5) will decrease coverage by 5.69%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop      #63      +/-   ##
===========================================
- Coverage    91.77%   86.07%   -5.70%     
===========================================
  Files            6        6              
  Lines          158      158              
===========================================
- Hits           145      136       -9     
- Misses          13       22       +9     

Impacted Files	Coverage Δ
src/models/vd.jl	80.76% <0.00%> (-11.54%)	⬇️
src/input/tools.jl	90.90% <0.00%> (-9.10%)	⬇️
src/models/call_powermodels.jl	92.98% <0.00%> (-5.27%)	⬇️
src/input/pp_to_pm.jl	75.00% <0.00%> (-2.28%)	⬇️

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