ci: update Github Action workflows

[nicrie]

New structure separates workflows into
separate workflows: ci, PR linting, black formatting and creating
release. The latter is only triggered
after a PR has been merged. All others are triggered when
a PR on main/develop is opened, edited or synchronized.

[codecov-commenter]

Codecov Report

Merging #73 (7919090) into develop (8e199ef) will increase coverage by 5.70%.
Report is 62 commits behind head on develop.
The diff coverage is 97.54%.

@@             Coverage Diff             @@
##           develop      #73      +/-   ##
===========================================
+ Coverage    91.46%   97.17%   +5.70%     
===========================================
  Files           39       65      +26     
  Lines         2600     4382    +1782     
===========================================
+ Hits          2378     4258    +1880     
+ Misses         222      124      -98     

Files Changed	Coverage Δ
xeofs/utils/constants.py	100.00% <ø> (ø)
xeofs/preprocessing/_base_stacker.py	73.91% <50.00%> (-2.28%)	⬇️
xeofs/models/_base_model.py	78.04% <73.91%> (+3.04%)	⬆️
xeofs/utils/rotation.py	88.67% <77.77%> (ø)
xeofs/preprocessing/_base_scaler.py	81.81% <80.00%> (-2.40%)	⬇️
xeofs/utils/statistics.py	68.00% <85.71%> (ø)
xeofs/utils/xarray_utils.py	92.53% <87.09%> (-2.30%)	⬇️
xeofs/preprocessing/stacker.py	93.00% <88.59%> (-0.96%)	⬇️
xeofs/preprocessing/scaler.py	91.40% <91.56%> (+3.55%)	⬆️
xeofs/data_container/_base_model_data_container.py	92.15% <92.15%> (ø)
... and 55 more