Fix: valu and debug engine literal missing

[zhangddjs]

as original defined engine only has ["alu", "load", "store", "flow"]

Engine = Literal["alu", "load", "store", "flow"]

Which should be conflict against given example and ENGINE_FNS:

"""
Here's an example of what an instruction might look like:

{"valu": [("*", 4, 0, 0), ("+", 8, 4, 0)], "load": [("load", 16, 17)]}
"""

ENGINE_FNS = {
    "alu": self.alu,
    "valu": self.valu,    # <-- here valu engine missing
    "load": self.load,
    "store": self.store,
    "flow": self.flow,
}

So I decided to add the missing "valu" and "debug" engine

-- Engine = Literal["alu", "load", "store", "flow"]
++ Engine = Literal["alu", "valu", "load", "store", "flow", "debug"]