support `Shiki` transformers

[Christian Preston]

Transformers

• twoslash
  • Example: https://github.com/antfu/nuxt-content-twoslash

[Pooya Parsa]

shiki.highlight (API in nuxt-shiki) allows transformers to be passed (via programmatic also component) in 0.2+

I think it should be enough to unblock any higher order usages of this module to include transformers.

Additionally we could support a plugin hook to allow extending default transforms. I think specially since this module could be used by multiple modules, global mutations are inheritingly a dangerous territory to support (a plugin hook that adds transform for all highlighters)