Make rst code blocks work correctly with newer Vim patch levels.

Vim's runtime grep support for highlighting code blocks in
reStructuredText recently.  Unfortunately, our implementation and the
new implementation collide causing 'c' code blocks to highlight past the
end of the code block.

It turns out that the new implementation also embeds @NoSpell into the
code blocks, which we don't care for either.  Let's redo the work, this
time avoiding @NoSpell.  While we're at it, let's make this work for
patch levels that don't have rstCodeBlock already.