fix: use Bash 5.3 compgen -V to generate completions including newlines

[akinomyoga]

As mentioned in #704 (comment), @algorythmic has added compgen -V array_name in the new version of Bash 5.3, which can be used to handle the completions including newline characters properly.

• 7c12535 In the first commit, I split the function _comp_compgen into two (for _comp_compgen <generator> and for _comp_compgen -- <options for builtin compgen>).
• 0ec92f2 In the second commit, I prepare an implementation for Bash 5.3 using compgen -V.

edit: Although the CI tests with Bash 5.3 are not set up, I locally ran tests with Bash 5.3 (more specifically, with the devel ranch of Bash) and confirmed that this change does not break the tests.