I didn't think it made sense, but I found something that used it,
so apparently it's a thing. This updates the operand editor to
let you choose PETSCII+DCI, and updates the assemblers to handle
it correctly (really just 64tass, since the others either don't
have a DCI directive or don't deal with PETSCII at all).
Changed the char-encoding sample from "bad dcI" to "pet dcI", and
updated the documentation.
All tests use the same data file and nearly the same project file.
The only difference is the default text encoding property setting.
For "-a" it's ASCII, for "-p" it's PETSCII, for "-s" it's C64 screen
code. Right now this only affects the code generated for 64tass.
The test itself is a collection of strings and characters in the
supported character encodings. How these are handled varies
significantly between assemblers.
