minimal zsh completion (#20253)

Since zsh can load bash completion files natively, seems reasonable to just turn this on.
The only changes are to switch from `type -t` which zsh doesn't support to using `type`
with a regex and adding a new arm to the sourcing of the completions to allow it to work
for zsh as well as bash.

Could use more bash/dash/etc testing probably, but everything I've thought to try has
worked so far.

Notes:
* unit-test zsh support, fix issues
Specifically fixed word splitting in completion-test, use a different
method to apply sh emulation to zsh loaded bash completion, and fixed
an incompatibility in regex operator quoting requirements.

* compinit now ignores insecure directories
Completion isn't meant to be enabled in non-interactive environments, so
by default compinit will ask the user if they want to ignore insecure
directories or load them anyway.  To pass the spack unit tests in GH
actions, this prompt must be disabled, so ignore explicitly until a
better solution can be found.

* debug functions test also requires bash emulation
COMP_WORDS is a bash-ism that zsh doesn't natively support, turn on
emulation for just that section of tests to allow the comparison to
work.  Does not change the behavior of the functions themselves since
they are already pinned to sh emulation elsewhere.

* propagate change to .in file

* fix comment and update script based on .in

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