Add better shell scripting capabilities

[vinc]

At the moment we can only run one command at a time in the REPL and in any shell script, which is pretty limiting. This PR will evolve the shell into a proper scripting language inspired by Ruby allowing us to do much more with it:

• Add ; to separate commands
• Add and operator
• Add or operator
• Add not operator
• Add test program
• Add def <name> <args>* [do] <commands> end (function definition)
• Add if <condition> [do] <commands> [else <commands>] end (conditional)
• Add for <var> in <list> [do] <commands> end (loop)
• Add $(<command>) (command substitution)
• Add <command> --> <command> (pipe)

Example:

def hello name
  print "Hello, $name!"
end

if test -e $USER
  hello World
else
  hello $USER
end

for file in $(list /tmp/*.tmp)
  drop $file
end

A real shell script could be a redefinition of move as copy + drop instead of being a dedicated Rust program:

#! shell

def usage
  print "Usage: move <src> <dst>"
end

for arg in $*
  if test -e $arg "-h" or test -e $arg "--help"
    usage
    exit 0
  end
end

set src $1
set dst $2

if test -e $src "" or test -e $dst ""
  usage
  exit 1
end
        
copy $src $dst and drop $src

[vinc]

Command substitution has been implemented in #822