Add handleCompletePromise helper function

[avillega]

We had some code repetition around handling a completed durable promise. The repetition was fine, how ever this pattern will come to callbacks and tasks and we will have it in atleast 4 more places, having to write that code that many times is error prone and verbose.

This change also fixes a bug made evident by this helper funciton where we were returning the value computed by the function directly instead of the value stored in the completed durable promise.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.09%. Comparing base (86725f3) to head (f4e5d84).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/core/promises/types.ts	84.61%	2 Missing ⚠️
lib/resonate.ts	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
+ Coverage   79.94%   81.09%   +1.15%     
==========================================
  Files          17       17              
  Lines        1107     1095      -12     
  Branches      300      314      +14     
==========================================
+ Hits          885      888       +3     
+ Misses        222      195      -27     
- Partials        0       12      +12     

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