6502: Make the shifted PETSCII characters conveniently available without having to manually re-draw them

[susan-davis]

PETSCII has a wealth of line and box drawing characters that are super useful, and are already there in character ROM on C64 family hardware. (And Unicode defines them in its Symbols for Legacy Computing block.) But we don't get them available out of the box when mkfont runs, and have to re-create them from scratch. Having them available as standard options to throw in, without having to re-draw them pixel-for-pixel, would be nice.

Stretch goal: make the extra characters from unshifted PETSCII available, too.

Turning around and having a (presumably monospace) font available on hand to use in the web interpreter for these characters would be cool, too, along with some sort of convenient syntax in Dialog for using them.

[dstelzer]

On the mkfont side, this seems straightforward enough: just look up the shapes of the PETSCII characters and put them in the font definition file. On the Dialog side, though, it'll take a bit more work: Symbols for Legacy Computing is in the Supplementary Multilingual Plane, and while the Å-machine supports that, Dialog currently doesn't. Hopefully that will be fixed soon.