Replace DurablePromise union type with single type

[avillega]

Implements the proposed change in
#126

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.47619% with 4 lines in your changes missing coverage. Please review.

Project coverage is 82.44%. Comparing base (7bf23ef) to head (4a05b1a).

Files	Patch %	Lines
lib/core/stores/remote.ts	81.25%	3 Missing ⚠️
lib/core/storages/withTimeout.ts	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
- Coverage   82.63%   82.44%   -0.19%     
==========================================
  Files          22       22              
  Lines        1399     1401       +2     
  Branches      381      356      -25     
==========================================
- Hits         1156     1155       -1     
+ Misses        243      240       -3     
- Partials        0        6       +6     

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[avillega]

The slight reduction in code coverage is expected.