Can `--install-if-missing` be added to `fnm env` so that `--use-on-cd` can work without prompting

[nemchik]

Currently with fnm env --use-on-cd

• If I cd into a directory that has a file defining a node version I do have, this works as expected by switching to the node version defined.
• If I cd into a directory that has a file defining a node version I do not have have, I am prompted to install that version.
  • Correction: After the install is complete it is not automatically used.

I would like:

• If I cd into a directory that has a file defining a node version I do not have have, I would like fnm to automatically install and use that version without prompting me.

I imagine this behavior is not wanted by everyone, so I'm hoping this can be a separate flag so that people can opt to use it.