Vimwiki reader: Add nested syntax highlighting (#6257)

Nested syntaxes are specified like this:
{{{sql
SELECT * FROM table
}}}

The preformatted code block parser has been extended to check if the
first attribute of the block is not a `key=value` pair, and in that case
it will be considered as a class.

Closes #6256.

1 files changed, 6 insertions, 0 deletions

diff --git a/test/vimwiki-reader.wiki b/test/vimwiki-reader.wiki
index c85d663ae..01973a41a 100644
--- a/test/vimwiki-reader.wiki
+++ b/test/vimwiki-reader.wiki
@@ -117,6 +117,12 @@ Just two backticks: ``
      print(i)
  }}}
 
+=== preformatted text with nested syntax ===
+
+{{{sql
+SELECT * FROM table
+}}}
+
 === empty preformatted text ===
 
 {{{