Add support for Textile footnote backlinks (#8585)

Backlinking from footnotes is already the behavior of `Note` values
when writing HTML. This adds parsing support for the explicit
backlinking syntax in Textile as described here:
https://textile-lang.com/doc/footnotes

Previously, these lines would be interpreted as a new paragraph
beginning with "fn3^." in text.

src/Text/Pandoc/Readers/Textile.hs

@@ -90,7 +90,10 @@ parseTextile = do
   Pandoc nullMeta . B.toList <$> parseBlocks -- FIXME
 
 noteMarker :: PandocMonad m => TextileParser m Text
-noteMarker = skipMany spaceChar >> string "fn" >> T.pack <$> manyTill digit (char '.')
+noteMarker = do
+  skipMany spaceChar
+  string "fn"
+  T.pack <$> manyTill digit (string "." <|> try (string "^."))
 
 noteBlock :: PandocMonad m => TextileParser m Text
 noteBlock = try $ do

test/textile-reader.textile

@@ -269,7 +269,7 @@ A note.[1]  Another note[2].
 fn1. The note
 is here!
 
-fn2. Other note.
+fn2^. Other note.
 
 h1. Comment blocks