LaTeX reader: support \underline, \ul, \uline (#5359)

These are parsed as a Span with class `underline`, as with other readers.

src/Text/Pandoc/Readers/LaTeX.hs

@@ -866,6 +866,7 @@ inlineCommands = M.union inlineLanguageCommands $ M.fromList
   , ("slash", lit "/")
   , ("textbf", extractSpaces strong <$> tok)
   , ("textnormal", extractSpaces (spanWith ("",["nodecor"],[])) <$> tok)
+  , ("underline", underlineSpan <$> tok)
   , ("ldots", lit "…")
   , ("vdots", lit "\8942")
   , ("dots", lit "…")
@@ -1122,6 +1123,10 @@ inlineCommands = M.union inlineLanguageCommands $ M.fromList
   , ("foreignlanguage", foreignlanguage)
   -- include
   , ("input", include "input")
+  -- soul package
+  , ("ul", underlineSpan <$> tok)
+  -- ulem package
+  , ("uline", underlineSpan <$> tok)
   -- plain tex stuff that should just be passed through as raw tex
   , ("ifdim", ifdim)
   ]