refactor how extensions are loaded

[disberd]

This PR changes how extensions are loaded for target packages.

Since #50, we now have our target package loaded as root_module, which gives access to some more convenience functionality from Base.

Prior to this PR, the extensions code was parsed and loaded in the target module with custom functions defined in the package.

Now, the extensions are loaded by calling Base.insert_extension_triggers and Base.run_extension_callbacks.
To allow reloading extensions upon manual reload, we hackily remove the loaded extension from Base.EXT_PRIMED and from Base.loaded_modules so that it can be re-inserted by Base.run_extension_callbacks.

This should now support all the extensions functionality from julia itself, closing #39

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1be5847) to head (c8131c4).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #55   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          820       771   -49     
=========================================
- Hits           820       771   -49     

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