fix: add `core.json` to `package.json` `exports`

[SimenB]

Allows https://github.com/browserify/resolve/commit/122bb64be2f4fcf927fd264d4b2c9ea622611c58 to be just require('is-core-module/core.json')

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (77d33fb) 90.47% compared to head (83cb795) 90.47%.

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.