fix(text/unstable): make dedent match reference for multiline substitutions

[WWRS]

fixes #6665

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.69%. Comparing base (2e6ac51) to head (ba644a6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6667      +/-   ##
==========================================
- Coverage   94.69%   94.69%   -0.01%     
==========================================
  Files         562      562              
  Lines       46648    46651       +3     
  Branches     6569     6570       +1     
==========================================
+ Hits        44175    44177       +2     
- Misses       2430     2431       +1     
  Partials       43       43              

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[kt3k]

LGTM. Nice!