fix(markdown-remark): reuse shiki highlighter instance

[ocavue]

Changes

Closes #15656

I noticed the following warning in my Astro project:

[Shiki] 10 instances have been created. Shiki is supposed to be used as a singleton, consider refactoring your code to cache your highlighter instance; Or call `highlighter.dispose()` to release unused instances.
[Shiki] 20 instances have been created. Shiki is supposed to be used as a singleton, consider refactoring your code to cache your highlighter instance; Or call `highlighter.dispose()` to release unused instances.
[Shiki] 30 instances have been created. Shiki is supposed to be used as a singleton, consider refactoring your code to cache your highlighter instance; Or call `highlighter.dispose()` to release unused instances.

This is due to the fact that Astro creates a new Shiki highlighter instance for each language, and there are many different code block languages in my project.

This PR improves this by using the same highlighter across languages. This approach works because Astro already loads languages dynamically when they haven't been loaded yet (code link).

Testing

Added a new test to ensure that

1. The same highlighter instance is used for different languages. I not only cache the Shiki highlighter instance, but I also cache the returned wrapper object (code link) for an extra tiny bit of performance improvement.
2. This cached highlighter instance can highlight new languages.

Docs

Changeset file has been added.

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[ematipico]

This is wasteful because we're polluting the key with empty objects. I suggest looking for a different approach e.g.

const keyCache = [];
if (options?.theme) { keyCache.push(options.theme) }
...

const key = keyCache.length > 0 ? JSON.stringify() : "";

[ocavue]

done