Support BRR samples without loop headers

[PhoenixBound]

CoilSnake currently assumes all BRR samples have an AddMusicK-style header at the beginning (two bytes with the loop point). It's possible to check for this header by seeing whether len(brr_data) % 9 is 0 or 2, but CoilSnake doesn't check that:

CoilSnake/coilsnake/model/eb/musicpack.py

Lines 457 to 466 in 824dcc8

	# Load BRR data
	for inst, filename in zip(instruments, instrument_files):
	try:
	brr_raw_data: bytes = file_loader(filename, astext=False).read()
	brr_data = Block()
	brr_data.from_array(array('B', brr_raw_data))
	except FileNotFoundError:
	raise InvalidUserDataError("instrument BRR '{}' doesn't exist in instrument pack directory".format(filename))
	inst.sample = brr_data[2:]
	inst.sample_loop_offset = brr_data.read_multi(0, 2)

As a result, if the user takes the direct output of many ordinary BRR sample converters (Bregalad's brrtools excluding some forks, kleinesfilmroellchen's BRR converter, etc), CoilSnake silently corrupts them. At the very least, it should check file size invariants if only headered samples are supported. But since it's easy to check the format, it also might make sense to just support those samples and use a dummy loop point. (Maybe with a non-fatal warning message if the sample ends with a "loop" block?)