Muse reader: require that comment semicolons are in the first column

Fixes #4551

src/Text/Pandoc/Readers/Muse.hs

@@ -348,6 +348,7 @@ blockElements = do
 -- | Parse a line comment, starting with @;@ in the first column.
 comment :: PandocMonad m => MuseParser m (F Blocks)
 comment = try $ do
+  getPosition >>= \pos -> guard (sourceColumn pos == 1)
   char ';'
   optional (spaceChar >> many (noneOf "\n"))
   eol

test/Tests/Readers/Muse.hs

@@ -521,6 +521,7 @@ tests =
         , "Text after empty comment" =: ";\nfoo" =?> para "foo" -- Make sure we don't consume newline while looking for whitespace
         , "Not a comment (does not start with a semicolon)" =: " ; Not a comment" =?> para (text "; Not a comment")
         , "Not a comment (has no space after semicolon)" =: ";Not a comment" =?> para (text ";Not a comment")
+        , "Not a comment (semicolon not in the first column)" =: " - ; foo" =?> bulletList [para "; foo"]
         ]
       , testGroup "Headers"
         [ "Part" =: