feat(toolbox): add plugin to show `toolbox` prompt info (#10685)

plugins/toolbox/README.md

+# toolbox plugin
+
+Plugin for [toolbox](https://containertoolbx.org), a tool to use containerized CLI environments.
+
+To use it, add `toolbox` to your plugins array in your `.zshrc` file:
+
+```zsh
+plugins=(... toolbox)
+```
+
+## Prompt function
+
+This plugins adds `toolbox_prompt_info()` function. Using it in your prompt, it will show the toolbox indicator ⬢ (if you are running in a toolbox container), and nothing if not.
+
+You can use it by adding `$(toolbox_prompt_info)` to your `PROMPT` or `RPROMPT` variable:
+
+```zsh
+RPROMPT='$(toolbox_prompt_info)'
+```

plugins/toolbox/kubectx.plugin.zsh

+function toolbox_prompt_info() {
+  [[ -f /run/.toolboxenv ]] && echo "⬢"
+}