fix(helpers): add missing dir attribute for right-to-left languages

[lihbr]

Resolves: https://prismic-team.slack.com/archives/C014VAACCQL/p1729780207755199

Description

This PR updates the default richtext serializer to honor the direction property of rich text nodes and adds the appropriate dir attribute on right-to-left text nodes.

Related:

• @prismicio/client PR: fix(helpers): add missing dir attribute for right-to-left languages prismic-client#357
• @prismicio/react PR: fix(helpers): add missing dir attribute for right-to-left languages prismic-react#211

Checklist

• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

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[angeloashmore]

Perfect 👌

It's interesting to compare the simplicity of the Svelte implementation to the React and HTML versions. It's the most recent implementation—maybe there's something about it we should port to the other packages.