Fix/Verify parser for `DEC` instruction

[MalteT]

2a-emulator/emulator-2a-lib/src/compiler.rs

Line 184 in 741632e

_ => unimplemented!("DEC [something other than R*] does not work yet"),

The above may cause panics. This might need some verification from the actual machine, but making DEC only available for registers should be an easy fix.