Use IM_ASSERT instead of assert.h directly

[serivesmejia]

ImGui assertion callbacks were recently added to the imgui-java binding (SpaiR/imgui-java#100) to make debugging easier. However, this callback doesn't get triggered with imnodes assertion errors, a quick glance over the source code reveals that imnodes is using assert.h directly instead of using the IM_ASSERT macro which is configurable, is there any possibility of making this change so debugging becomes easier in java and possibly other wrappers? Thanks!