refactor(codegen): support 'prerelease' or 'developmode' in dfns

[wpbonelli]

Accommodate attribute "prerelease" rename to "developmode" in MODFLOW-ORG/modflow6#2556

[codecov]

Codecov Report

❌ Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.6%. Comparing base (556c088) to head (a01442e).
⚠️ Report is 76 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/codegen/filters.py	0.0%	16 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2635      +/-   ##
===========================================
+ Coverage     55.5%    72.6%   +17.0%     
===========================================
  Files          644      667      +23     
  Lines       124135   129063    +4928     
===========================================
+ Hits         68947    93709   +24762     
+ Misses       55188    35354   -19834     

Files with missing lines	Coverage Δ
flopy/mf6/utils/generate_classes.py	17.5% <ø> (ø)
flopy/mf6/utils/codegen/filters.py	0.0% <0.0%> (ø)

... and 557 files with indirect coverage changes

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