Define InputCharResult to distinguish between read-character, end-of-stream, and waiting for input states
Replace the boolean "isRunning" property with InterpreterState enum
Change Interpreter state-machine so that it can be driven by BitsyBASIC's main thread
Change INPUT implementation so that it properly handles the waiting-for-input case
Set BitsyBASIC working directory to the user's Documents directory (so that SAVE and LOAD will work)
Change user input field to green text on black background
Add description of BitsyBASIC target