Add config flag to exclude "made-with-panda" CSS Var

[rsmidt]

I've just stumbled across the "--made-with-panda" CSS Var that is added to the global styles.

IMO, this is unnecessarily polluting the global variables. Searching through the docs, I couldn't find a way to disable this behavior.

[Gravy59]

It's only ~25 bytes total though

[rsmidt]

Still, it's not needed to provide the necessary function, and it's basically user tracking without opt-out. Currently, I need to manually filter it out.