various fixes for footnotes

[QuietMisdreavus]

This PR adds a handful of fixes and improvements surrounding footnotes:

• Footnote definition and reference nodes now properly report a node type in cmark_node_get_type_string.
• XML output now emits the footnote IDs for footnote definitions and references.
• The "literal" string for footnote definitions (which holds the ID for the footnote) is now cached after the length adjustment that trims the trailing ]: from the text, allowing cmark_chunk_to_cstr to be called on these literals correctly.
• A new API, cmark_node_get_footnote_id has been added which returns the footnote ID for footnote definition and reference nodes. This can be used to easily load the authored footnote ID so that they can be matched with each other.

These changes are necessary for adding footnote support to Swift-Markdown and Swift-DocC.