Adds repository field to package.json.

[qubyte]

Adding this field means that:

• the next release will add a link to this repo in the sidebar of https://www.npmjs.com/package/@remy/webmention (good for auditing code and creating issues)
• the npm repo command is enabled
• npm logging a warning that no repo is defined is avoided

[vercel]

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[remy]

Cheers. I have a habit of doing npm init -f before making the git repo!

[qubyte]

No worries. I always forget it too 😅