src: simplify is_callable by making it a concept

[tniessen]

Using a C++20 concept here makes is_callable much simpler than relying on SFINAE. It is equivalent for function types, std::function, lambdas, and classes with operator(), regardless of argument or return types.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 90.14%. Comparing base (5fb879c) to head (33a9784). :warning: Report is 953 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #58169   +/-   ##
=======================================
  Coverage   90.13%   90.14%           
=======================================
  Files         630      630           
  Lines      186611   186611           
  Branches    36631    36634    +3     
=======================================
+ Hits       168204   168220   +16     
+ Misses      11192    11183    -9     
+ Partials     7215     7208    -7     

Files with missing lines	Coverage Δ
src/req_wrap-inl.h	92.72% <ø> (ø)
src/util.h	91.22% <ø> (ø)

... and 25 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/68630/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/68825/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/69206/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/69284/

[nodejs-github-bot]

Landed in f6d6b911fa3deb74e59f3cd9a5bb8dfecceabec4