Add custom `add_heir_....` CMake macros

[AlexanderViand-Intel]

We cannot use the existing add_mlir_{dialect/conversion/transform/analysis} CMake macros as they assume that source files are organized in a specific pattern, which is different from what we do in HEIR. However, it should be relatively straight-forward to write our own custom CMake macro that does the same thing, just with our directory structure in mind.