feat: toc generator

[aryan02420]

resolves #4

I am unable to get mdbox/parsers to initialize in the test runner. Until then I have written a basic toc generator.

[aryan02420]

This link is broken. The ast for this node is

[
  {
    "type": "h2",
    "children": [
      "heading with footnote",
      {
        "type": "footnoteReference"
      }
    ]
  },
  {
    "type": "footnoteDefinition",
    "children": [
      {
        "type": "p",
        "children": ["this is a footnote"]
      }
    ]
  }
]

The slug generated by this plugin is #heading-with-footnote while github generates #heading-with-footnote1