[TF-810] Reduce client bundle size for nextjs themes

[riffbyte]

Key changes:

• Re-arranged code to make more sense structure-wise, and to isolate the code that should only be used on server-side. Some parts of the code still get bundled to the client, but it looks like the only proper way to solve that would be to migrate to ESM modules, which is a task in of an itself. 21 Kb saved
• Get rid of string-strip-html library in favor of a new SDK about_plaintext property. 50 Kb saved
• Sitemap props fetching helper is now intended to be used in getServerSideProps, not getInitialProps

[linear]

TF-810 Reduce bundle size for NextJS Themes

This doesn't look good:

image.png

Need to check if some server-only code is leaking into the client bundle (this was the main offender in the bundle size for the website).