feat: cleanup & tweaks, vol.2

[robertrossmann]

This is the final round of "random" tweaks and improvements before I move on to rewriting the README.

• refactored the Observer struct into an interface and provided the DefaultObserver as an implementation of it
• moved Task struct and related functionality into a separate file
• implemented Manager.Close() that is a helper function which calls both .Wait() and .Cancel() in parallel. Useful to completely shut this thing down all at once.
• minor tweaks to the CI definition, plus added a lint step to the pipeline. 🎨

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 98.37%. Comparing base (691ccee) to head (cdbbef8).

Files	Patch %	Lines
background.go	95.65%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           feat/looping-tasks       #9      +/-   ##
======================================================
+ Coverage               98.13%   98.37%   +0.24%     
======================================================
  Files                       3        3              
  Lines                     107      123      +16     
======================================================
+ Hits                      105      121      +16     
  Misses                      2        2              

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