RFC: Drop Prism.js support

[antfu]

Slidev was created when Shiki was node-only. Since then, Shiki has evolved a lot and is superior in many ways. Having two highlighters at the same time not only makes maintenance harder, constrains the innovations on better integrations but also introduces extra effort for theme authors to offer themes for different highlighters.

This issue is a proposal to drop Prism support in future breaking changes - if you might be affected by this, please let us know your use cases, and we could see how we can find a middle ground for that.