[Enhancement] Highlight quotes in the shell part.

[YuanfengZhang]

As we know, the right quote in the shell part can be {input.foo}, {output.bar}, {params.xxx}, {wildcards.yyy}, {threads} ... Users might type {foo} in the shell script but there are no hint or highlight there, make the learning / debugging difficult.

expected:
highlight all the legal quotes between {} in the .smk files.
A simple regex example (python) to capture them could be this:
\{((?:input\.|output\.|threads|params\.|wildcards\.)\b)[^{}]*\}

Kinda ugly, never mind.