src: constrain COW forwarding constructors

[Archkon]

Exclude the wrapper type from the forwarding constructors so that non-const lvalue copies select the copy constructor.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.17%. Comparing base (c8e2a82) to head (efc5fa9).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64861      +/-   ##
==========================================
+ Coverage   90.16%   90.17%   +0.01%     
==========================================
  Files         746      746              
  Lines      242760   242768       +8     
  Branches    45765    45759       -6     
==========================================
+ Hits       218875   218907      +32     
+ Misses      15375    15347      -28     
- Partials     8510     8514       +4     

Files with missing lines	Coverage Δ
src/node_threadsafe_cow.h	100.00% <ø> (+8.33%)	⬆️

... and 34 files with indirect coverage changes

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