Update MD053/link-image-reference-definitions to handle references within references (i.e., Pandoc-style footnotes), verify Pandoc footnote indent is not treated as comment when markdown-it-footnote plugin is used (fixes #599).

test/pandoc-footnotes.md

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+# Pandoc Footnotes
+
+> Examples taken from [GitHub issue 599](https://github.com/DavidAnson/markdownlint/issues/599)
+
+## Example with Pandoc Footnotes
+
+A sentence with footnotes: [^1] [^2]
+A sentence with named footnotes: [^name] [^name2]
+A sentence with a link reference: [Pandoc's User Guide][Pandoc1]
+
+[^1]: I am a footnote!
+[^2]: I reference a [PCW][PCW1] article!
+[^name]: I am a footnote with name!
+[^name2]: I am also a named footnote! I also reference the [PCW][PCW1] article!
+
+[PCW1]: https://www.example.com/article.html
+[Pandoc1]: https://pandoc.org/MANUAL.html#extension-footnotes
+
+## Example with Long Pandoc Footnotes
+
+A sentence with a long footnotes: [^long] [^longer] [^longest]
+
+[^long]: I am a long footnote!
+    I don't do any harm :)
+[^longer]: I am a longer footnote. I do reference the [PCW][PCW2] article.
+    I do harm. Though, not here: [Pandoc's User Guide][Pandoc2]
+[^longest]: I am the longest footnote. I also reference the [PCW][PCW2] article.
+
+    I am a harmful new block of text: [Another][Another2]
+
+> The previous line of text is treated by CommonMark as an indented code block.
+> To handle it as a Pandoc footnote, consider the `markdown-it-footnote` plugin.
+
+[PCW2]: https://www.example.com/article.html
+[Pandoc2]: https://pandoc.org/MANUAL.html#extension-footnotes
+[Another2]: https://www.example.com/another.{MD053}.html
+
+## GitHub Footnotes
+
+Sample footnotes [^3] [^note3]
+
+[^3]: A line
+  A new line
+
+[^note3]:
+    I am a new block of text
+    With a new line as well