RP6502 EhBASIC graphic-plotting support - two screen dimensions

Demonstration-code of initial graphic-plotting support to EhBASIC described at https://github.com/picocomputer/ehbasic/issues/2#issuecomment-1868562547.

New 10-Jan-2023: two screen dimensions are supported.

Four new commands are available using EhBasic's "CALL" keyword. Plotting command callable from EhBASIC.
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:

• 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.

The Basic demo/test code includes:

-BarnsleyFern.txt
-Mandelbrot_set - minimal-touch port from Applesoft BASIC listed on Rosettacode.org