[checks] Add check for trailing `\br` in a `bnf` environment

[eisenwave]

It's quite easy to make a mistake such as:

\begin{bnf}
\nontermdef{line-directive}\br
    \terminal{\# line} pp-tokens new-line\br
\end{bnf}

If I recall correctly, this trailing \br is not ignored, but simply renders as an unwanted blank line. I have made this error during 2025 post-Sofia motions I believe.

It seems like it would be fairly easy to grep for \br, followed by whitespace,
followed by \end{bnf}.