Initial plotting enhancements coupled with save/load.

Working plotting-enhancements coupled with the 12-14-Jan-2024 EhBASIC+save/load mods from the master picocomputer EhBASIC repo.

Four new commands are available using EhBasic's "CALL" keyword. CALL addresses supplied are from the mapfile. Plotting command is callable from EhBASIC.

Two plotting screen dimensions are supported.

The commands are:

HGR,screen_dimension - initializes graphics screen mode
    CALL HGR,0 - selects 320h x 180v x 8bpp mode (16:9 letterbox);
    CALL HGR,$FF - selects 320h x 240v x 4bpp mode (4:3 fullscreen)
HPLOT,x,y,color - paint a pixel of 'color' at x,y on the screen.
TEXTMODE - return VGA-screen back to console/text mode
CLS (or HOME) - clear the console/text screen.

Assumptions / Limitations / Notes:

HPLOT targets (only) the two screen-modes listed above on the rp6502's pico-VGA.
The x-coordinate from EhBasic is limited to values <= 255 (8-bits). This is a
limitation of parameters following the 'CALL' EhBASIC keyword.
EhBASIC's executable's origin-address: $C000.
48639-bytes reported available to the user.

Discussion-thread with example graphics plots.