Split Prestel from Beeb emulation parsing, add VDU 7, 23, and 31

Prestel and Beeb emulations are different enough that using a shared
switch statement doesn't really make sense.

This adds Mode 7 support for VDU commands 7 (beep), 23 (cursor only),
and 31 (move to X/Y, 0-based).

The rest of the unsupported commands don't seem overly useful in a
BBS context (disable/enable output) or I don't want to deal with
documenting and implementing ("Page mode").