feat(env): add --install-if-missing flag to --use-on-cd

[pd4d10]

Summary

Fixes #1452

• Adds --install-if-missing flag to fnm env that passes --install-if-missing to the fnm use call in the generated --use-on-cd shell hook
• When set, both the initial inline fnm use (at eval time) and the cd hook will automatically install the target Node version if it's not already present
• The flag is gated with requires = "use_on_cd" so it can only be used together with --use-on-cd

Usage

eval "$(fnm env --use-on-cd --install-if-missing)"

[changeset-bot]

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