Add support for Composer version 2

[Ayesh]

Hi John,

Composer version 2 has composer-plugin-api version 2. This PR updates the version constraint to ^1 || ^2 so we can support both composer versions.

See What's new in Composer 2 and UPGRADE-2.0 for more changes in API. Empty methods \johnpbloch\Composer\WordPressCorePlugin::deactivate() and \johnpbloch\Composer\WordPressCorePlugin::uninstall() are added to make it compatible both versions.

Related: composer/composer#8726

Thank you.

[codecov-io]

Codecov Report

Merging #27 into master will not change coverage. The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master      #27   +/-   ##
=========================================
  Coverage     94.59%   94.59%           
  Complexity       19       19           
=========================================
  Files             2        2           
  Lines            37       37           
=========================================
  Hits             35       35           
  Misses            2        2           

Impacted Files	Coverage Δ	Complexity Δ
src/johnpbloch/Composer/WordPressCorePlugin.php	60.00% <0.00%> (ø)	3.00 <2.00> (ø)

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[johnpbloch]

Howdy @Ayesh,

Thanks so much for taking the time to send this PR in! I ended up causing a few conflicts with your work in the process of getting the test suite to work with the latest version of PHPUnit, so I'm not sure how it's going to end up attributing the line changes to you when I resolve them and merge this in, but just in case I also gave you text credit in the changelog at the bottom of the readme.

[Ayesh]

Awesome, thank you so much 🙏.