Provide tree of headings in `@island/headings`

**Is your feature request related to a problem? Please describe.**
Since the `@islands/headings` packages provides headings as a list, instead of a tree, I am unable to render a proper tree in the table of contents.

**Describe the solution you'd like**
I would prefer that during parsing of the AST, headings be compiled into a tree-structure instead of (or in addition to) a list so that the relationships between headings is also available in the `post.meta` field.

**Describe alternatives you've considered**
This could be accomplished by a separate plugin but a lot of the code would be duplicated between the `@islands/heading` module and the separate one.

**Additional context**
Having the headings as a tree would make it possible to index the headings and provide meaningful numbering to them as well. For example, something like this would be possible if the headings were a tree.

```
§1 Chapter
§1.1 Section
§1.2 Section
§1.2.1 Subsection
§2 Chapter
...
```

I'm happy to make a PR if this change seems useful to the general user base.

Locate the @islands/headings AST parser and inspect how headings are exposed through post.meta. Clarify the tree shape and whether the existing list should remain available, then cover nested headings such as chapters, sections, and subsections; done means consumers can access their heading relationships for table-of-contents indexing and numbering.

Written by the indexing model from the issue text.