Update isdigit, isalpha, and isalnum to handle Unicode characters

[sjanzou]

Addresses issue #53

Windows debug build in VS 2022 with no assert errors

[cpaulgilman]

Thank you for the fix! Windows Debug works without assertion failure, and Linux Release works as expected. Note LK IDE in Linux shows unknown characters as question mark (?) while Windows (above) shows a random character. Screenshot from Linux Mint 21.1 Cinnamon:

[cpaulgilman]

@sjanzou I added a commit so that the following LK script does not result in a debug assert error:

x = isdigit('');
outln(x);

Please change if there's a better way to do that.

[sjanzou]

@sjanzou I added a commit so that the following LK script does not result in a debug assert error:

x = isdigit('');
outln(x);

Please change if there's a better way to do that.

Thanks for checking for null characters.