Pass filename to highlighter

[vnphanquang]

Context

Today, as of mdsvex@0.11.0, a custom highlighter function is not aware of the filename of the file being processed by MDsveX, and there is no way (that i am aware of) to get around this using the public API.

This issue adds such support by passing additional filename positional param for highlighter.

Rationale

Having filename in highlighter will allow more complex customization. Specifically, I'm trying to have the syntax highlighter read the source code from file system provided in a meta line, something like this:

```svelte
/// source=./examples/an-example.svelte
```

This allows keeping certain example source code in a separate file, making better use of editor support, enabling reusability of said source code, and keeping the MDsveX file succinct.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 939981d07051b96ba28846058f34ee99d4ba55fd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ------ | ----- | | mdsvex | Patch |

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[vnphanquang]

Ran prettier just now, amended to existing commit. Sorry for not doing that earlier.

[pngwn]

Sorry for the delay! Released now https://github.com/pngwn/MDsveX/releases/tag/mdsvex%400.11.1

[vnphanquang]

Sorry for the delay! Released now https://github.com/pngwn/MDsveX/releases/tag/mdsvex%400.11.1

Thanks 🎉. No worries at all