Support escaped `|` in table cells

[matloob]

Here's a program that runs goldmark and markdown on the following input https://go.dev/play/p/YgApD-obwxL

Foo | Bar
--- | ---
A   | a\\|b\\|c

rendered in github as

Foo	Bar
A	a\|b\|c

The output of the program is

markdown:
<table>
<thead>
<tr>
<th>Foo</th>
<th>Bar</th>
</tr>
</thead>
<tbody>
<tr>
<td>A</td>
<td>a\</td>
</tr>
</tbody>
</table>

goldmark:
<table>
<thead>
<tr>
<th>Foo</th>
<th>Bar</th>
</tr>
</thead>
<tbody>
<tr>
<td>A</td>
<td>a\|b\|c</td>
</tr>
</tbody>
</table>