Offer fnm as an installation option on all platforms

[MattIPv4]

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Enter your suggestions in details:

fnm is a Node version manager (similar to NVM) designed to be fast with full cross-platform support for macOS/Linux/Windows. I think it would be beneficial to add it to the site as an option for folks looking to download/install a Node.js version on any platform.

https://formulae.brew.sh/formula/fnm shows 22k installs in the last year via brew, for some context on usage. https://formulae.brew.sh/formula/nvm sits at 457k installs in the same period.

[MattIPv4]

I believe this tangentially relates to nodejs/package-maintenance#591

[ovflowd]

@MattIPv4 feel free to open a PR.