Add warning for (list)string_var or (list)key_var in LSO

[Sei-Lisa]

This implements the warning suggested in #35.

Here's a test case that triggers it:

default
{
    state_entry()
    {
        key k = llGetObjectName();
        string s = llGetKey();
        list lst =
                   (list)    // $[E20021] could result in a string
                         k;
        lst = lst +
                    (list)         // $[E20021] could result in a key
                          s;
        if (llGetListEntryType(lst, 0) == TYPE_STRING
         && llGetListEntryType(lst, 1) == TYPE_KEY)
        {
            // This is printed!
            llOwnerSay("LSO has weird bugs");
        }
    }
}

If the above script is compiled with Mono mode disabled, it will output "LSO has weird bugs".

Contains also a couple very minor changes which are not worth a PR.

[Sei-Lisa]

Sorry, I've force-pushed a minor fix (change of the internal name of the warning, and change a comment in the Mono script)