feat(octicons-react): add files to package.json and update npmignore

[joshblack]

Update the package.json and .npmignore for the @primer/octicons-react package. Specifically,

• Add the files array to package.json to include only the files needed for publish
• Update the .npmignore file to match on patterns currently being included in the package
• Add the exports field to package.json with the import and require keys

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 09b5a779e311003c9ee0ceec93930d16413daccc

The changes in this PR will be included in the next version bump.

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