Draft: pmb.config.init: allow accepting default answers

This merge request addresses #2072.

• Adds an accept_default parameter to the pmb.helpers.cli.ask function so that the question is output and the default answer is used.
• Sets accept_default to True for all init questions when the -y or --assume-yes flag is set.
• Updates the .ci/prepare.sh script to use --assume-yes.

Tested by running yes "" | ./pmbootstrap.py init and ./pmbootstrap.py -y init and confirming that pmbootstrap.cfg is the same for both.