Refactor proper name formatting to reduce indents

[dafeda]

Pre review checklist

• Read through the code changes carefully after finishing work
• Make sure tests pass locally (after every commit!)
• Prepare changes in small commits for more convenient review (optional)
• PR title captures the intent of the changes, and is fitting for release notes.
• Updated documentation
• Ensured that unit tests are added for all new behavior (See
  Ground Rules),
  and changes to existing code have good test coverage.

Pre merge checklist

• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.

[codecov-commenter]

Codecov Report

Merging #6201 (c3802e7) into main (68ff10b) will decrease coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #6201      +/-   ##
==========================================
- Coverage   82.66%   82.66%   -0.01%     
==========================================
  Files         347      347              
  Lines       21248    21248              
  Branches      833      833              
==========================================
- Hits        17565    17564       -1     
- Misses       3385     3387       +2     
+ Partials      298      297       -1     

Files	Coverage Δ
src/ert/validation/proper_name_argument.py	94.73% <100.00%> (ø)
src/ert/validation/proper_name_format_argument.py	94.73% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[kvashchuka]

Nice!

[eivindjahren]

👍