This commit changes "deno repl" command to run with no permissions by
default and accept "--allow-*" flags.
This change is dictated by the fact that currently there is no way to
run REPL with limited permissions. Technically it's a breaking
change in the CLI command, but there's agreement in the team
that it has merit and it's a good solution.
Running just "deno" command still starts the REPL with full permissions
allowed, but now a banner is printed to inform users about that:
