PyBasic ported to Circuitpython

Changes for compatibility with Vintage Basic / MS Basic varients

• Print element separator changed to ;
• Print will not print cr/lf if final token is ;
• Input prompt / variable separator changed to ;
• RND required argument. Negative values reseed with that value, Zero returns last value (not implemented), any positive number returns random float between 0 and 1
• DIM allows multiple array definitions seperated by commas
• DIM over dim by 1 to handle dialetcs that are 1 based and expect there to be an element = length

New functions / keywords

• FREE : reports free memory at prompt
• TONE freq duration : plays a tone of frequency for duration
• CURSOR x y : moves the terminal cursor to X/Y
• GETCHR : waits for a single keypress from the keyboard. Returns the ascii value of key
• CLR : clears the screen, works in a statement or at prompt