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/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"