Convention register are not handled correctly

[AndoniZubimendi]

If the name of the register starts with $ it is assumed to be valid always if it is only one letter and a number, and returns the $0 register.

It also does not check for uppercase names too.

daddi $j0 $0, 150
daddi $S0, $0, 100
halt

The string $S0 and $j5 are resolved as register $0, not giving any error

Pass 1 completed with 0 errors
00000000          .code
00000000 6000000a daddi $j0, $0, 10
00000004 6000000a daddi $S0, $0, 10
00000008 04000000 halt

Pass 2 completed with 0 errors
Code Symbol Table
Data Symbol Table