nginx-syntax: support highlighting for variables with curly braces

[Iridias]

nginx supports specifying variables with curly braces, similar to bash (e.g.: ${var})
This change will make kate highlight them as well.

[mtorromeo]

Hi, sorry about the long wait to handle this and thanks for the contribution.
I would improve this by matching the closing bracket only if the opening one was matched before (using back-references).