fix: add core.json to package.json exports

[SimenB]

Allows browserify/resolve@122bb64 to be just require('is-core-module/core.json')

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.47%. Comparing base (77d33fb) to head (83cb795).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #14   +/-   ##
=======================================
  Coverage   90.47%   90.47%           
=======================================
  Files           1        1           
  Lines          42       42           
  Branches       17       17           
=======================================
  Hits           38       38           
  Misses          4        4           

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[ljharb]

While this is certainly a technically viable resolution for resolve, I am very very very loath to add to the exposed API of is-core-module.