refactor(spellfile): config() interface, docs #36481

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|.

runtime/lua/vim/_meta/options.lua

@@ -6527,7 +6527,7 @@ vim.bo.spf = vim.bo.spellfile
 --- encoding is used, Vim doesn't check it.
 --- How the related spell files are found is explained here: `spell-load`.
 ---
---- If the `spellfile.vim` plugin is active and you use a language name
+--- If the `spellfile.lua` plugin is active and you use a language name
 --- for which Vim cannot find the .spl file in 'runtimepath' the plugin
 --- will ask you if you want to download the file.
 ---