diagnostics_channel: return original thenable

[Qard]

This makes tracePromise return the original thenable to allow custom thenable types to retain their methods rather than producing the chained result type.

This branches the behaviour between native promises and thenables such that native promises retain the correct unhandledRejection behaviour while thenables produce the correct original return value and so retain methods present on that type.

I think that because native promises are consistently shaped it doesn't actually matter that it remains chained rather than branched.

cc @nodejs/diagnostics

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (2263b4d) to head (3eb07ea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #62407   +/-   ##
=======================================
  Coverage   89.68%   89.69%           
=======================================
  Files         676      676           
  Lines      206710   206720   +10     
  Branches    39584    39585    +1     
=======================================
+ Hits       185398   185417   +19     
- Misses      13441    13442    +1     
+ Partials     7871     7861   -10     

Files with missing lines	Coverage Δ
lib/diagnostics_channel.js	98.52% <100.00%> (+0.03%)	⬆️

... and 28 files with indirect coverage changes

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[Renegade334]

Does this change have a specific case in mind? The A+ standard mandates .then() to return another "promise" instance (either the same object or a new object at the implementation's discretion), so any custom features of a compliant thenable should also be present on the object returned by .then()?

[Renegade334]

require('internal/util/types').isPromise()?