Define/configure `:syntax sync match`

[blueyed]

I have noticed that when scrolling larger files with @media sections, the syntax highlighting might be off, i.e. a closing brace is highlighted as an error.

This might be related to setting up/configuring :syntax sync match ... in the syntax file (see :h :syn-sync-fourth), but I am not sure and just stumbled upon this.

I am using both syntax/css.vim and syntax/sass.vim from the development branches.