feat(helmfile): introduce helmfile plugin

plugins/helmfile/README.md

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+# Helmfile plugin
+
+This plugin adds completion and aliases for [Helmfile](https://helmfile.readthedocs.io/), the declarative helm charts deployer.
+
+To use it, add `helmfile` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... helmfile)
+```
+
+## Aliases
+
+| Alias |  Full command          |
+| ----- | ---------------------- |
+| hf    | helmfile               |
+| hfi   | helmfile --interactive |

plugins/helmfile/helmfile.plugin.zsh

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