Create structure for footnote references.

src/parser/footnote_reference.rs

@@ -0,0 +1,13 @@
+use super::Context;
+use crate::error::Res;
+use crate::parser::FootnoteReference;
+use crate::parser::util::not_yet_implemented;
+
+#[tracing::instrument(ret, level = "debug")]
+pub fn footnote_reference<'r, 's>(
+    context: Context<'r, 's>,
+    input: &'s str,
+) -> Res<&'s str, FootnoteReference<'s>> {
+    not_yet_implemented()?;
+    todo!()
+}