Problem:
- Exposing the raw config as table is a pattern not seen anywhere else
in the Nvim codebase.
- Old spellfile.vim docs still available, no new documentation
Solution:
- Exposing a `config()` function that both acts as "getter" and "setter"
is a much more common idiom (e.g. vim.lsp, vim.diagnostic).
- Add new documentation and link old docs to |spellfile.lua| instead of
|spellfile.vim|.
Problem:
Spell file downloads relied on Vimscript and netrw (:Nread). If netrw is
disabled, downloads fail.
Solution:
Port the logic to Lua as `nvim.spellfile` and wire it via a Lua plugin that
handles `SpellFileMissing`. Use `vim.net.request()` with a timeout for HTTP,
prompt via `vim.fn.input` and report via `vim.notify`.
Closes#7189
