Refine "blending" rules for MediaWiki links

The rules for "blending" characters outside a link into the link are
described here: https://en.wikipedia.org/wiki/Help:Wikitext#Blend_link

These pose a problem for CJK languages, which generally don't have
spaces after links.

However, it turns out that the blending behavior, as implemented on
Wikipedia, is (contrary to the documentation) only for ASCII letters.
This commit implements that restriction, which fixes the problem for
CJK.  (jgm#8525)

src/Text/Pandoc/Readers/MediaWiki.hs

@@ -19,7 +19,7 @@ module Text.Pandoc.Readers.MediaWiki ( readMediaWiki ) where
 
 import Control.Monad
 import Control.Monad.Except (throwError)
-import Data.Char (isDigit, isSpace)
+import Data.Char (isAscii, isDigit, isLetter, isSpace)
 import qualified Data.Foldable as F
 import Data.List (intersperse)
 import Data.Maybe (fromMaybe, maybeToList)
@@ -664,7 +664,7 @@ internalLink = try $ do
              -- [[Help:Contents|] -> "Contents"
              <|> return (B.text $ T.drop 1 $ T.dropWhile (/=':') pagename) )
   sym "]]"
-  linktrail <- B.text <$> manyChar letter
+  linktrail <- B.text <$> manyChar (satisfy (\c -> isAscii c && isLetter c)) 
   let link = B.link (addUnderscores pagename) "wikilink" (label <> linktrail)
   if "Category:" `T.isPrefixOf` pagename
      then do