Add support for Pragmas. A pragma can be added by the addition of a comment at the end of a line. The comment has the format "# Pragma: xxxxx" where xxxxx is the name of a specific supported pragma. Each pragma, when enabled changes the way in which the line is translated in some way.

Initial support is included for a nullish coalescing pragma which changes all '||' operators to '??' within a line. It is enabled with a '??' pragma.

For example:

a ||= b # Pragma: ??

This results in the translation:

a ??= b;