Fix MDX heading IDs generation when using a frontmatter reference

[HiDeoo]

Changes

This PR fixes #5780.

When using a frontmatter reference in a MDX heading, the ID generated for the heading is not using the reference value. For example,

---
title: The Title
---

# {frontmatter.title}

would generate <h1 id="frontmattertitle">The Title</h1> instead of <h1 id="the-title">The Title</h1>.

This pull request updates the @astrojs/markdown-remark rehype plugin generating these IDs to fix this behavior:

1. For MDX document, if the heading is a MDXTextExpression, e.g. # {frontmatter.title}, the plugin will use the associated ESTree entry to get the variable path, e.g. ['frontmatter', 'title'].
2. If the variable is referencing a frontmatter value, the plugin try to get the value from the Astro data containing the frontmatter.
3. If the value is found, the plugin will use it to generate the heading ID.

Note: the added dev dependencies to @astrojs/markdown-remark are only used to reference types.

Testing

I added a new fixtures test-with-frontmatter.mdx to the @astrojs/mdx test suite to test the changes.

The tests covers various cases and syntaxes from a basic variable to nested list items:

---
title: The Frontmatter Title
keywords: [Keyword 1, Keyword 2, Keyword 3]
tags:
  - Tag 1
  - Tag 2
  - Tag 3
items:
  - value: Item 1
  - value: Item 2
  - value: Item 3
nested_items:
  nested:
    - value: Nested Item 1
    - value: Nested Item 2
    - value: Nested Item 3
---

# {frontmatter.title}
## frontmatter.title
### {frontmatter.keywords[1]}
### {frontmatter.tags[0]}
#### {frontmatter.items[1].value}
##### {frontmatter.nested_items.nested[2].value}
###### {frontmatter.unknown}

Docs

I could not find any specific mention regarding this specific behavior in the docs and I'm not sure if it's worth mentioning it.

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Latest commit: 05ec52f

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[natemoo-re]

Changes look good to me but want to pull @bholmesdev in since I know he's been working with MDX a lot lately

[bholmesdev]

A few nits, but amazing work. Thorough checks and unit testing here. An IDeal fix 👏