Change `assert` macro to `IM_ASSERT`

[DenSinH]

I want to use this module in a project of mine, but I defined my own IM_ASSERT. ImNodes seems to just use assert from <assert.h>, but I think it would make more sense if it would also use IM_ASSERT, like the rest of ImGui. The fix should be easy, I was able to literally find and replace assert by IM_ASSERT in ImNodes.cpp.

[rokups]

Sure thing 👍🏻