refactor: revert default content to live in host MFE

[jsnwesson]

This PR extracts the default component logic from env.config.js to live in the Host MFE, and thus wrapped by the Plugin Slot. This is necessary in order to ensure that core functionality remains in the Host MFE rather than defined in a JS config, which would have impacted the developer experience for engineers who weren't planning on using the plugin slot.

The primary changes include:

• keepDefault boolean replaces default_contents array in JS config
• Default Content has a priority of 50 so any plugins can be inserted before/after
• Updated example apps to render default content accordingly
• Added Plugin Slot example for when keepDefault is set to false
• Default Content section of README is updated

APER-3281

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 96.53%. Comparing base (1fe6e1b) to head (2bce431).
Report is 2 commits behind head on master.

Files	Patch %	Lines
src/plugins/PluginSlot.jsx	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   94.57%   96.53%   +1.95%     
==========================================
  Files          10       10              
  Lines         166      173       +7     
  Branches       32       35       +3     
==========================================
+ Hits          157      167      +10     
+ Misses          7        5       -2     
+ Partials        2        1       -1     

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[MaxFrank13]

This is great. Thanks for getting all this done. I just had a handful of nit picks. I'm going to approve now but am happy to discuss anything further 👍