feat(anyspawn): Make Spawner usable without features

[AdomasBekeras]

To use Spawner in a library, the library should not need to enable any features, it can just take a Spawner. This PR removes the feature gates from the Spaner and JoinHandle types

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (8b9c195) to head (af2ce27).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #343   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         210      210           
  Lines       15540    15553   +13     
=======================================
+ Hits        15540    15553   +13     

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[martintmk]

I suggest to also have empty default features.