feat: fields and message expansion feature

[pamburus]

Description

This pull request adds the ability to expand fields and messages.

  -x, --expansion <MODE>      Whether to expand fields and messages [env: HL_EXPANSION=] [default: medium] [possible values: never, inline, low, medium, high, always]

• Expansion mode never completely disables field expansion, even if multiline fields are found.
• Expansion mode inline expands only multi-line fields as-is (inline) without any prefixes or formatting.
• Expansion mode low uses low expansion settings profile, which minimizes expansion.
• Expansion mode medium uses medium expansion settings profile, which provides an optimal balance between readability and density.
• Expansion mode high uses high expansion settings profile, which shifts the balance towards readability.
• Expansion mode always expands every field regardless of its value and expands the message if it is multiline.

The default mode is now medium. To keep the old behavior, use the inline mode.

Demo

[codecov]

Codecov Report

Attention: Patch coverage is 98.30724% with 40 lines in your changes missing coverage. Please review.

Project coverage is 76.73%. Comparing base (b582b3e) to head (890e328).

Files	Patch %	Lines
src/app.rs	94.62%	17 Missing ⚠️
src/cli.rs	93.20%	7 Missing ⚠️
src/formatting.rs	99.55%	6 Missing ⚠️
src/model.rs	80.95%	4 Missing ⚠️
crate/heapopt/src/lib.rs	97.61%	3 Missing ⚠️
crate/mline/src/lib.rs	99.28%	2 Missing ⚠️
src/fmtx.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
+ Coverage   72.04%   76.73%   +4.68%     
==========================================
  Files          40       42       +2     
  Lines       10533    12484    +1951     
==========================================
+ Hits         7589     9580    +1991     
+ Misses       2944     2904      -40     

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