Add nullchar constant, NULLCHAR feature, and vectrex encoding

include/string.mfk

@@ -25,7 +25,7 @@ word strz2word(pointer str) {
     errno = err_ok
     while true {
         char = str[i]
-        if char == 0 {
+        if char == nullchar {
             if i == 0 {
                 errno = err_numberformat
             }