Add support for the shebang in scripts

[jmmv]

EndBASIC scripts should be allowed to start with a Unix-style shebang (#!/usr/bin/env endbasic).

At first, it seems like this should be implemented in the CLI when loading files... but given that # is not a valid character in BASIC, that'd prevent sharing those files with the web for example. So maybe we need to implement this at a lower level.