Fix issue 311: add an 'undo' command

[dennisvang]

@JMSwag This is a first attempt at alleviating issue #311.

There's probably lots of room for improvement, and I'm not sure if this covers all edge cases, but it seems like a good start.

This PR includes tests and documentation.

[codecov[bot]]

Codecov Report

Merging #312 (23fbb1d) into main (7093d76) will decrease coverage by 0.93%. The diff coverage is 92.11%.

:exclamation: Current head 23fbb1d differs from pull request most recent head 6c0f255. Consider uploading reports for the commit 6c0f255 to get more accurate results

@@            Coverage Diff             @@
##             main     #312      +/-   ##
==========================================
- Coverage   88.14%   87.20%   -0.94%     
==========================================
  Files          26       26              
  Lines        2336     2392      +56     
==========================================
+ Hits         2059     2086      +27     
- Misses        277      306      +29     

Impacted Files	Coverage Δ
pyupdater/cli/commands.py	87.75% <87.14%> (-0.56%)	:arrow_down:
pyupdater/client/__init__.py	89.25% <88.88%> (+0.88%)	:arrow_up:
pyupdater/core/package_handler/package.py	80.00% <89.65%> (-2.45%)	:arrow_down:
pyupdater/client/patcher.py	83.58% <90.47%> (+0.90%)	:arrow_up:
pyupdater/client/updates.py	74.54% <92.85%> (-15.65%)	:arrow_down:
pyupdater/core/package_handler/__init__.py	90.82% <94.87%> (-0.38%)	:arrow_down:
pyupdater/cli/options.py	100.00% <100.00%> (ø)
pyupdater/core/key_handler/keys.py	83.17% <100.00%> (ø)
pyupdater/core/package_handler/patch.py	86.79% <100.00%> (ø)
pyupdater/settings.py	100.00% <100.00%> (ø)
... and 9 more

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[dennisvang]

Changed to draft, pending merge of PR #322