Implement EDIT statement and ON TIMER/ON KEY event trapping, update to v0.9.0

Add event-driven programming: ON TIMER(n) GOSUB with TIMER ON/OFF/STOP,
ON KEY(n) GOSUB with KEY(n) ON/OFF/STOP for F1-F10. Fix F-key escape
sequence parser (F9/F10 detection, push back consumed bytes on unmatched
sequences). Add EDIT statement for TUI line editing. Guard key trap
polling so keystrokes aren't consumed when no traps are configured.

tests/programs/on_timer.bas

@@ -0,0 +1,12 @@
+10 REM ON TIMER event trapping test
+20 COUNT = 0
+30 ON TIMER(1) GOSUB 100
+40 TIMER ON
+50 T0 = TIMER
+60 IF TIMER - T0 < 3 THEN 60
+70 TIMER OFF
+80 PRINT "Timer fired"; COUNT; "times"
+90 IF COUNT >= 2 THEN PRINT "PASS" ELSE PRINT "FAIL"
+95 END
+100 COUNT = COUNT + 1
+110 RETURN