feat(molecule): introduce molecule plugin (#12760)

plugins/molecule/README.md

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+# Molecule plugin
+
+This plugin adds completion for [Molecule](https://ansible.readthedocs.io/projects/molecule/), the project designed to aid in the development and testing of Ansible roles..
+
+To use it, add `molecule` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... molecule)
+```

plugins/molecule/molecule.plugin.zsh

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+# Completion
+if (( ! $+commands[molecule] )); then
+  return
+fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `molecule`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_molecule" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _molecule
+  _comps[molecule]=_molecule
+fi
+
+_MOLECULE_COMPLETE=zsh_source molecule >| "$ZSH_CACHE_DIR/completions/_molecule" &|