trs80: implement Grid shim in video driver

Loading the grid subsystem on top of the TRS-80 driver, which is
an interface to its ROM BIOS, would be counter-productive because
the BIOS already has the concept of cursor.

However, VE needs the Grid interface, so we implement a shim for it.

Again, all this work (and the re-assembly that it entails) has been
done from within Collapse OS on the TRS-80!

arch/z80/trs80/blk.fs

@@ -29,6 +29,8 @@ CODE AT-XY EXX, ( protect BC )
 EXX, ( unprotect BC ) ;CODE
 ( ----- 604 )
 : LINES 24 ; : COLS 80 ;
+: XYMODE 0x70 RAM+ ;
+: CELL! COLS /MOD AT-XY (emit) ;
 CODE BYE
     HL 0 LDdi,
     A 0x16 LDri, ( @EXIT )