Implement a function to detect the end of a footnote reference definition with balanced brackets.

src/parser/footnote_definition.rs

@@ -67,7 +67,7 @@ pub fn footnote_definition<'r, 's>(
 }
 
 #[tracing::instrument(ret, level = "debug")]
-fn label<'s>(input: &'s str) -> Res<&'s str, &'s str> {
+pub fn label<'s>(input: &'s str) -> Res<&'s str, &'s str> {
     alt((
         digit1,
         take_while(|c| WORD_CONSTITUENT_CHARACTERS.contains(c) || "-_".contains(c)),