DVItext1: Add rudimentary terminal code handling

[jepler]

The following are handled:

ESC [ K               ; clear to end of line
ESC [ 2 J             ; clear screen
ESC [ count D         ; cursor backwards (default = 1)
x08                   ; cursor backwards 1 (ctrl-H)
ESC [ row ; column H  ; position cursor
ESC [ attr m          ; 0 = normal, 1 = inverse
ESC ] ; ... x1B x5c   ; sets title (ignored)
x07                   ; bell (flashes screen) (ctrl-G)

"ESC[ m" is used by RunCPM to set some characters to bold, implemented here as reverse video. The rest are the ones implemented in CircuitPython terminalio and so probably represent a minimal useful collection of codes.

Bell is used by RunCPM for instance when backspacing on an empty input line. The flash is similar to the visual bell used in some terminal applications.

The 1bit_text demo is enhanced so that as soon as a character is printed to the Serial connection, it switches to typewriter mode. This can be used to test escape code handling.

[jepler]

An alternative approach would be to make the base class NOT handle complex codes and allow derived classes or encapsulating classes to do so. In the CP/M emulator, having ADM-3 terminal emulation may be more useful.

[PaintYourDragon]

Leaning toward the alternative approach thoughts. Rather than tying DVItext1 to a specific terminal implementation (which then makes others problematic), maybe one can register a terminal-specific callback function that write() then uses as a filter on output. Or, if doing derived classes, something that works with multiple inheritance to combine a terminal type and a display type (idea here is that we might eventually have a DVItext4 or something, to handle color output, I just haven’t gotten the Round Tuit yet).