Add farcaster to social links

[dionysuzx]

Description

This PR should allow 'farcaster' to be used as an option in the social icons schema. This is so that starlight users can add a social link to their farcaster site (eg. warpcast) like so:

social: {
    ...
    farcaster: 'https://warpcast.com/@someone`
}

Do I need to add a changeset..?

[changeset-bot]

🦋 Changeset detected

Latest commit: 1c235ea

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

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Patch

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[delucis]

Thanks @dionysuzx! I believe this was the intention when someone first added the icon, but I guess we forgot to add the social schema part.

We’ll need a changeset so this is reflected in our changelog, but I can add one now before merging.