Test for \footnotemark and \footnotetext (issue #11450)

```
% pandoc -f latex -t native
Text\footnotemark{}.
\footnotetext{The footnote content.}
^D
[ Para
    [ Str "Text"
    , Note
        [ Para
            [ Str "The"
            , Space
            , Str "footnote"
            , Space
            , Str "content."
            ]
        ]
    , Str "."
    ]
]
```

With explicit footnote numbers:

```
% pandoc -f latex -t native
First\footnotemark[1] and second\footnotemark[2].
\footnotetext[1]{First note.}
\footnotetext[2]{Second note.}
^D
[ Para
    [ Str "First"
    , Note [ Para [ Str "First" , Space , Str "note." ] ]
    , Space
    , Str "and"
    , Space
    , Str "second"
    , Note [ Para [ Str "Second" , Space , Str "note." ] ]
    , Str "."
    ]
]
```

Mixed with regular footnotes:

```
% pandoc -f latex -t native
Text\footnotemark[1] and more\footnote{Regular footnote.}
\footnotetext[1]{Marked footnote.}
^D
[ Para
    [ Str "Text"
    , Note [ Para [ Str "Marked" , Space , Str "footnote." ] ]
    , Space
    , Str "and"
    , Space
    , Str "more"
    , Note [ Para [ Str "Regular" , Space , Str "footnote." ] ]
    ]
]
```
