feat(plugins): add fnm plugin (#9864)

Co-authored-by: Ziga Sebenik <ziga.sebenik@oryxgaming.com>

plugins/fnm/.gitignore

+_fnm
+

plugins/fnm/README.md

+# fnm plugin
+
+This plugin adds autocompletion for [fnm](https://github.com/Schniz/fnm) - a Node.js version manager.
+
+To use it, add `fnm` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... fnm)
+```

plugins/fnm/fnm.plugin.zsh

+# COMPLETION FUNCTION
+if (( $+commands[fnm] )); then
+  if [[ ! -f $ZSH_CACHE_DIR/fnm_version ]] \
+    || [[ "$(fnm --version)" != "$(< "$ZSH_CACHE_DIR/fnm_version")" ]] \
+    || [[ ! -f $ZSH/plugins/fnm/_fnm ]]; then
+      fnm completions --shell=zsh > $ZSH/plugins/fnm/_fnm
+      fnm --version > $ZSH_CACHE_DIR/fnm_version
+  fi
+  autoload -Uz _fnm
+  _comps[fnm]=_fnm
+fi
+