Docx reader: Parse hyperlinks in instrText tags

This was a form of hyperlink found in older versions of word. The
changes introduced for this, though, create a framework for parsing
further fields in MS Word (see the spec, ECMA-376-1:2016, §17.16.5,
for more on these fields).

Closes #3389 and #4266.

src/Text/Pandoc/Readers/Docx.hs

@@ -446,8 +446,10 @@ parPartToInlines' (PlainOMath exps) =
   return $ math $ writeTeX exps
 parPartToInlines' (SmartTag runs) = do
   smushInlines <$> mapM runToInlines runs
-parPartToInlines' (Field _ runs) = do
-  smushInlines <$> mapM runToInlines runs
+parPartToInlines' (Field info runs) = do
+  case info of
+    HyperlinkField url -> parPartToInlines' $ ExternalHyperLink url runs
+    UnknownField -> smushInlines <$> mapM runToInlines runs
 parPartToInlines' NullParPart = return mempty
 
 isAnchorSpan :: Inline -> Bool