Merge MANIFOLD_EXCEPTIONS and MANIFOLD_DEBUG

[elalish]

Currently our readme says our MANIFOLD_DEBUG flag controls both assertions and exceptions, but in fact we have a second CMake flag: MANIFOLD_EXCEPTIONS, which defaults to ON. However, nearly all of our exceptions use DEBUG_ASSERT, for which both debug and exceptions must be enabled to fire. Only Vec uses ASSERT (and hull, but I think that's a mistake) which is enabled even when debug is off. I can't imagine a situation where an app is going to be able to do much if we throw a Vec assertion - it's basically just a seg-fault, and it certainly shouldn't happen if we've written our code properly.

I propose we merge MANIFOLD_EXCEPTIONS into MANIFOLD_DEBUG and use only a single kind of ASSERT. This will have the effect of turning exceptions off by default, which seems right? Feedback welcome - and WDYT @pca006132?

[pca006132]

Sounds good to me.