find: implement {} path substitution for -exec

[Erickkaranja]

A utility_name or argument containing only the two characters "{}" shall be replaced by the current pathname.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.73%. Comparing base (89a5327) to head (63326a3).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/find/matchers/exec.rs	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #620      +/-   ##
==========================================
- Coverage   91.76%   91.73%   -0.03%     
==========================================
  Files          31       31              
  Lines        6203     6207       +4     
  Branches      328      329       +1     
==========================================
+ Hits         5692     5694       +2     
- Misses        390      391       +1     
- Partials      121      122       +1     

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