Add support for declaring aliases for internal global variables

[DannyBen]

cc #586, #581

This PR adds these settings:

var_aliases:
  args: ~
  other_args: ~
  deps: ~
  env_var_names: ~

which will declare aliases using declare -gn to allow, for example, to use $ARGS instead of $args, or $catch_all instead of $other_args.