docs: improve Neovim/Vim setup instructions

[SchoolyB]

Summary

The current Neovim setup instructions in the README are a bare nvim-lspconfig Lua snippet. They should be expanded to give users a smoother setup experience.

Current State

A single Lua config block with a hardcoded path (/Users/you/code/grayls/out/server.js) and no surrounding context.

Suggested Improvements

• Prerequisites (Node.js, building the server with npm run build)
• Step-by-step setup for nvim-lspconfig users
• Filetype detection for .gray files (vim.filetype.add)
• Syntax highlighting options (Tree-sitter grammar at grayscale-lang/tree-sitter-gray)
• Troubleshooting (verifying the server starts, checking :LspLog)

[SchoolyB]

Implemented in 2706b1c. Two notes on the description:

The section was further along than "Current State" suggests. It was not a single Lua block — it already had three labelled steps (install nvim-lspconfig, configure the server, register the filetype) and a note about rebuilding and :LspRestart. I expanded it rather than replacing it, keeping the existing nvim-lspconfig snippet intact.

What was actually missing, and is now covered:

• Prerequisites — build the server first, plus the Node version and the gray-on-$PATH requirement for diagnostics.
• The hardcoded /Users/you/... path, replaced with /path/to/grayls/out/server.js and an explicit note to substitute it.
• The built-in vim.lsp.config / vim.lsp.enable path for Neovim 0.11+, since the lspconfig.configs API is the older approach. Both are documented.
• Tree-sitter setup and highlight queries.
• Troubleshooting: :checkhealth vim.lsp, verifying the filetype resolved to gray, running the server by hand, :LspLog, and :echo exepath('gray') for the diagnostics-only failure mode.

One correction to the suggested scope: tree-sitter-gray does ship queries/highlights.scm — it is not parser-only. But nvim-treesitter does not install queries for third-party parsers, so users still have to copy it onto their runtimepath as queries/gray/highlights.scm. The README documents it that way.

Filetype detection via vim.filetype.add was already present; it is now Step 1, since the server silently fails to attach without it.