LaTeX reader: improve handling of newif.

Adding a pair of braces around the second argument of `\def`
prevents LaTeX from an emergency stop with:  Closes #6096.

```
pandoc -f markdown -o test.pdf
\newif\ifepub

\epubtrue

\ifepub

hi

\fi
^D
```

src/Text/Pandoc/Readers/LaTeX/Macro.hs

@@ -130,13 +130,17 @@ newif = do
                    Macro GroupScope ExpandWhenUsed [] Nothing
                    [ Tok pos (CtrlSeq "def") "\\def"
                    , Tok pos (CtrlSeq name) ("\\" <> name)
+                   , Tok pos Symbol "{"
                    , Tok pos (CtrlSeq "iftrue") "\\iftrue"
+                   , Tok pos Symbol "}"
                    ])
            , (base <> "false",
                    Macro GroupScope ExpandWhenUsed [] Nothing
                    [ Tok pos (CtrlSeq "def") "\\def"
                    , Tok pos (CtrlSeq name) ("\\" <> name)
+                   , Tok pos Symbol "{"
                    , Tok pos (CtrlSeq "iffalse") "\\iffalse"
+                   , Tok pos Symbol "}"
                    ])
            ]