Add shell variables

[vinc]

• Add access to the the pathname of the script or the shell via $0
• Add access to script arguments via $1, $2, $3, and so on
• Add access to environment variables like $HOME
• Allow setting a shell variable with test = 42 to access it via $test

[vinc]

The shell will create a var for each env var at launch but a subsequent call to env test=42 for example will not make a $test available like test = 42 would because env is not a built-in command at the moment.

[vinc]

Should we transform test = 42 into set test = 42 (or let test = 42) and add unset test?