Add Neovim configuration information

[RossSmyth]

The JSON provided for VSCode works with coc and nlsp-settings. This is verified by
https://github.com/fannheyward/coc-rust-analyzer#configurations for coc, and feeding the JSON in to this schema
https://github.com/tamago324/nlsp-settings.nvim/blob/main/schemas/_generated/rust_analyzer.json for validating nlsp-settings.

The Lua translation is straight-forward and is how all rust-analyzer settings must be used with native nvim lsp.

Note that I just moved and my main computer that has nvim setup on it is not unpacked so I did not test these. But I believe they should work, and I think I've actually done something like this in the past.

[RossSmyth]

May want to block merging, at least the Lua portion, until #1516 and/or #1542 are resolved

[RossSmyth]

I did test the nlsp-settings config and it accepted the config.

It still has some problems, but not with the config. Mainly rust-lang/rust-analyzer#8330 and some other issue I don't know. Making RA work with rustc is a struggle. But I have autocomplete, code actions, jumping to definitions, and type hints which is better than anything I got with VSCode lol.

[RossSmyth]

Actually it wasn't working, but it is because the JSON in the book is invalid. It has a trailing comma which is invalid in JSON. So it was causing nvim to send a null dict. But I fixed it and also made the steps clear as it was confusing to me (first open a buffer, then update RA settings)

[JohnTitor]

This is a pre-existing problem, but the explanation of VSCode and Neovim is mixed up and difficult to understand to me. Could you break down them as a sub-section of the "Configuring rust-analyzer for rustc"?

[RossSmyth]

This is a pre-existing problem, but the explanation of VSCode and Neovim is mixed up and difficult to understand to me. Could you break down them as a sub-section of the "Configuring rust-analyzer for rustc"?

Sounds good

[RossSmyth]

Also I cannot build these docs on my computer (Windows).

PS C:\Users\Ross\Documents\rustc-dev-guide> mdbook build
2023-02-09 16:37:13 [INFO] (mdbook::book): Book building has started
2023-02-09 16:37:14 [INFO] (mdbook::book): Running the html backend
2023-02-09 16:37:16 [INFO] (mdbook::book): Running the linkcheck backend
2023-02-09 16:37:16 [INFO] (mdbook::renderer): Invoking the "linkcheck" renderer
2023-02-09 16:37:16 [ERROR] (mdbook::utils): Error: Rendering failed
2023-02-09 16:37:16 [ERROR] (mdbook::utils):    Caused By: Unable to start the backend
2023-02-09 16:37:16 [ERROR] (mdbook::utils):    Caused By: %1 is not a valid Win32 application. (os error 193)

[jyn514]

This is great, thank you!