Remove legacy

[kjvalencik]

This PR includes lots of clean-ups--each as an independent commit--but, only ones that made sense to merge at once.

• Remove legacy (neon-sys, legacy backend, features, tests)
• After removing the backend, lots of dead code was left for it. Deleted the deadcode.
• Left with useless feature flags for napi-1. Remove these.
• After removing feature flags, lots of use statements were unnecessarily complicated. Cleaned all of the use statements for consistent style.
• Cleaned up tests, feature flags and aliases
• Moved neon-runtime to neon::sys. There isn't a reason for it to be a separate crate anymore and this will yield better performance with thin LTO.
• Removed other unused components: neon-build, cli

There are a few future clean-ups I would like to perform that are not included. For example:

• Re-organize repo. crates only has one crate now. Move src into crates/neon. Create npm directory (name?) move create-neon and cargo-cp-artifact into it.
• Update neon::sys (formerly neon-runtime) methods to be more idiomatic instead of the style of legacy. This is removing a lot of out pointers. Eliminate the build closure
• Rename napi everywhere to node-api and deprecate napi- feature flags
• Eliminate all uses of transmute
• Audit for methods that should be unsafe but, aren't. I've seen this most on new_internal and from_raw methods.
• Remove unnecessary context internals (e.g., isolate tracking)

[dherman]

This PR is so gratifying 🔥

A few questions and a couple minor comments, but it looks great.

[kjvalencik]

@dherman Thanks for reviewing! I added a commit to address the review feedback. Additionally, I rebased on top of main and fixed merge conflicts.

[dherman]

Looks great!