`*`, `_` surrounded by spaces should not trigger emphasis

[anntzer]

See https://pandoc.org/MANUAL.html#emphasis:

A * or _ character surrounded by spaces, or backslash-escaped, will not trigger emphasis: This is * not emphasized *, and \*neither is this\*.

This can also be confimed by running pandoc itself:

$ echo '**foo** ** foo **' | pandoc
<p><strong>foo</strong> ** foo **</p>

But currently vim-pandoc-syntax highlights **foo** and ** foo ** in the same way (both bold).

Edit: on a similar note, pandoc does not allow ** to span multiple paragraphs, but the syntax highlighter is not aware of that.