Add `--install-if-missing` option to fnm exec for missing Node versions

[hzy]

Problem

When running a command such as:

fnm exec --using=24.12.0 node -v

If the requested Node.js version is not installed yet, the command fails with the following error:

error: Requested version v24.12.0 is not currently installed

Proposal

It would be helpful to provide an option like --install-if-missing to automatically install the requested version if it is not present, similar to how fnm use behaves. This would improve the developer experience and reduce friction when running one-off commands using specific Node versions.

Example usage:

fnm exec --using=24.12.0 --install-if-missing node -v

This would install v24.12.0 if it is missing and then run the command as intended.

Motivation

• Consistent experience across fnm exec and fnm use
• Reduce manual installation steps
• Useful for scripting and CI/CD scenarios

Thank you for considering this enhancement!