latex: fix headings with curly braces inside them

The headings are delimited by the opening and closing curly braces. But
they can contain other latex commands inside them, for example:
\section{\LaTeX{}ila}
\section{\textit{italic} normal}

And the curly braces pairs can be nested:
\section{\textit{foo \textbf{bar}}}
latex.lang: do not highlight curly braces for headings

For all the other commands, the curly braces are not highlighted. So to
be consistent, do the same for the headings.
latex.lang: support *-forms headings

Example:
\section*{text}

The section won't be numbered, and won't appear in the table of
contents.