Help text improvements: group_tree_spacers, strict_usage_column_width

[dskrypa]

Added new configurable options:

• group_tree_spacers: Configure the characters that are used when show_group_tree is True
• strict_usage_column_width: Control line wrapping for the usage column in help text

Removed the min_usage_column_width config option, which was poorly conceived and defined, and was part of a buggy implementation of help text formatting that resulted in different usage column widths depending on nested group level when show_group_tree is True.

Now, usage_column_width is treated as a strict value that is not modified. Commands with deeply nested ParamGroups that use show_group_tree=True may benefit from increasing the usage_column_width value from its default.

The relatively internal format_help_entry function was changed, along with some related ParamHelpFormatter methods. The tw_offset parameter was removed (related to the removal of the min_usage_column_width config option), and cont_indent was renamed to usage_cont_indent for clarification. Other changes were made to the way the code related to format_help_entry is implemented to make it slightly easier to read/follow.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.32%. Comparing base (4bac88d) to head (f0f331e).
Report is 3 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #63   +/-   ##
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  Coverage   98.32%   98.32%           
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  Files          49       49           
  Lines        6616     6643   +27     
  Branches     1396     1400    +4     
=======================================
+ Hits         6505     6532   +27     
  Misses         95       95           
  Partials       16       16           

Flag	Coverage Δ
unittests	98.32% <100.00%> (+<0.01%)	⬆️

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