Support for selectively applying Expressive Code only to some Markdown documents.

We had a request in withastro/starlight#3220 to only apply Expressive Code to documents Starlight controls to allow code in other Markdown pages in a site to look different.

To do that, I suppose EC would need a configuration option to allow specifying the file paths it should process. For example an API something like:

expressiveCode({
  paths: ["src/content/docs/**"],
})

It would also open up the question (at least thinking about an Astro context) of whether multiple EC instances could make sense to have different styles for different parts of a site:

integrations: [
  expressiveCode({ paths: ["src/content/docs/**"], /* some config */ }),
  expressiveCode({ paths: ["src/content/blog/**"], /* some different config */ }),
],

That part is probably not so simple given considerations like the <Code> component, but wanted to share this here in case people have similar needs or thoughts.