Add zsh support to completion script

[gnodet]

Summary

• Add zsh support to the existing mvnd-bash-completion.bash script (auto-detects shell type)
• Accept zsh as valid --completion argument in Completion.java
• Update README.adoc with zsh instructions

Supersedes #1514 by consolidating the zsh compatibility into the single existing script rather than adding a separate file. This avoids duplication and maintenance burden.

Approach

The script auto-detects bash vs zsh and adapts three shell-specific behaviors:

• function_exists: uses typeset -f (zsh) vs declare -F (bash)
• Plugin variable discovery: uses typeset + (zsh) vs compgen -v (bash)
• Indirect variable expansion: uses ${(P)var} (zsh) vs ${!var} (bash)

For zsh, bashcompinit is loaded to provide bash-style completion builtins (complete, compgen, COMPREPLY). No global shell options (setopt) are modified.

Credit

Based on the approach from #1514 by @janweinschenker.

Test plan

• Full build passes
• Completion template regeneration via CompletionGeneratorTest succeeds
• Source the script in bash: works as before
• Source the script in zsh: completions work via bashcompinit layer

🤖 Generated with Claude Code

[janweinschenker]

What I tested on zsh:

$ mvnd [TAB]
# resulted in:

$ mvnd 
clean                    generate-sources         install                  post-integration-test    pre-site                 process-sources          site                     validate
compile                  generate-test-resources  integration-test         post-site                prepare-package          process-test-classes     site-deploy              verify
deploy                   generate-test-sources    package                  pre-clean                process-classes          process-test-resources   test                     
generate-resources       initialize               post-clean               pre-integration-test     process-resources        process-test-sources     test-compile  

$ mvnd cl [TAB]
# resulted in:
$ mvnd clean

$ mvnd clean pa [TAB]
# resulted in:
$ mvnd clean package

$ mvnd -pl m
# correctly gave me a list with all sub-modules

$ mvn -P [TAB][TAB]
# resulted in:
-1    -B    -C    -D    -N    -P    -T    -U    -V    -X    -am   -amd  -c    -cpu  -e    -emp  -ep   -f    -fae  -ff   -fn   -gs   -h    -l    -npr  -npu  -nsu  -o    -pl   -q    -rf   -s    -t    -up   -v