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.
The documentation for 64tass says you're required to pass "--ascii"
when the source file is ASCII (as opposed to PETSCII). We were
ignoring this, but it turns out that everything works a bit better
if we don't.
So we now pass "--ascii" on the command line, and add a two-line
character encoding definition to every file that is generated with
ASCII as the default encoding. The sg_petscii and sg_screen
encodings go away, as PETSCII is now the default, and we can use the
built-in "screen" encoding.
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.
