Add NIR_DEBUG env var

So far we have multiple NIR related debug env vars: NIR_TEST_CLONE, NIR_TEST_SERIALIZE, NIR_PRINT, and some others.

Let's merge them in an single NIR_DEBUG one, providing a help option to show all the available debug options. I'll keep the NIR_SKIP one in its own env var, but document it.

On top of that, add print_{cs,fs,gs,..} option to dump specific NIR shaders on each successful optimization/lowering call. Useful when interesting on see how a specific shader is being optimized/lowered.