proper setup for the fallback logger

[rvalieris]

fix for #268

the previous code was setting the log level of the root logger, and creating a fallback "jupyterlab" logger that inherits from root. also this was being set outside of any function/method so just by importing the file the log level changed.

since the root logger is not actually used, its better to set the level directly on the fallback logger and only if the passed on logger is not available which shouldn't happen anyway.

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[codecov-commenter]

Codecov Report

Merging #269 (d785432) into main (93703e2) will decrease coverage by 0.18%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #269      +/-   ##
==========================================
- Coverage   65.37%   65.18%   -0.19%     
==========================================
  Files          22       22              
  Lines        1730     1735       +5     
  Branches      295      296       +1     
==========================================
  Hits         1131     1131              
- Misses        515      520       +5     
  Partials       84       84              

Impacted Files	Coverage Δ
jupyterlab_server/process.py	0.00% <0.00%> (ø)

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[blink1073]

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