Implement DATE$/TIME$/TIMER, FILES, SHELL, CHDIR, MKDIR, RMDIR

DATE$, TIME$, and TIMER now return real system date/time instead of
hardcoded values. Added directory and shell access statements with
proper GW-BASIC error codes (Path not found 76, File already exists 60).

Bump to v0.6.0, 52 tests.

tests/programs/datetime.bas

@@ -0,0 +1,15 @@
+10 REM DATE$, TIME$, TIMER test
+20 D$ = DATE$
+30 T$ = TIME$
+40 R = TIMER
+50 IF LEN(D$) <> 10 THEN PRINT "FAIL date len": END
+60 IF MID$(D$,3,1) <> "-" THEN PRINT "FAIL date sep1": END
+70 IF MID$(D$,6,1) <> "-" THEN PRINT "FAIL date sep2": END
+80 IF LEN(T$) <> 8 THEN PRINT "FAIL time len": END
+90 IF MID$(T$,3,1) <> ":" THEN PRINT "FAIL time sep1": END
+100 IF MID$(T$,6,1) <> ":" THEN PRINT "FAIL time sep2": END
+110 IF R < 0 OR R > 86400 THEN PRINT "FAIL timer range": END
+120 PRINT "DATE$ = "; D$
+130 PRINT "TIME$ = "; T$
+140 PRINT "TIMER ="; INT(R)
+150 PRINT "All date/time tests passed"

tests/programs/filesystem.bas

@@ -0,0 +1,11 @@
+10 REM FILES, MKDIR, CHDIR, RMDIR test
+20 MKDIR "gwb_test_dir"
+30 OPEN "gwb_test_dir/test.txt" FOR OUTPUT AS #1
+40 PRINT #1, "hello"
+50 CLOSE #1
+60 CHDIR "gwb_test_dir"
+70 SHELL "pwd > /dev/null"
+80 CHDIR ".."
+90 KILL "gwb_test_dir/test.txt"
+100 RMDIR "gwb_test_dir"
+110 PRINT "All filesystem tests passed"