fix: remove unneeded "source" key from package.json exports

[nthurow]

Proposed changes

The "source" field in the package.json "exports" is causing issues with some repos. The purpose of this field is so that tools such as Jest and Storybook can resolve references to local packages without those packages needing to be built first. However, in our entire repository there are currently no references to @optum/react-hooks which would need to be resolved locally, and the existence of this "source" custom condition is conflicting with other repos that also use "source".

If we do at some point require this functionality (for example, if we turn this repository into a monorepo with multiple packages) then we should use a more unique name other than "source" to avoid conflicts with other repos that might be using "source" as a custom export condition.

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