Rewrite fnm in Rust (merge fnm.rs into fnm) — adding Windows support!

[Schniz]

As I already mentioned on Twitter, this PR will migrate fnm to a Rust implementation.

It started as an experiment but I'm using it for a while now and I think it's the right decision for fnm.

This is a big change. It's going to take some time to stabilize.
I'll comment here the big differences and what we're gaining from this move.

New Features

• 🔥 Windows support 🔥
  • Both PowerShell and WinCmd are supported, however only PowerShell have the same test coverage as Zsh/Bash/Fish
  • All features should work, including --use-on-cd

Breaking Changes

• --multi is now the default. I need to add a warning that this field is deprecated. Maybe to replace it with --single if we want to allow a single version.
• LTS aliases scheme has changed (latest-dubnium => lts-dubnium)

Internal Changes

• Stopped using DOM parsing for finding links, instead using index.json
• Feature tests are written in a higher-level "lisp"-y style, which generates test files for all the given shells.

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