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/plugin/nvim/spellfile.lua

@@ -1,16 +1,12 @@
+if vim.g.loaded_spellfile_plugin ~= nil then
+  return
+end
 vim.g.loaded_spellfile_plugin = true
 
---- Downloads missing .spl file.
----
---- @param args { bufnr: integer, match: string }
-local function on_spellfile_missing(args)
-  local spellfile = require('nvim.spellfile')
-  spellfile.load_file(args.match)
-end
-
 vim.api.nvim_create_autocmd('SpellFileMissing', {
-  group = vim.api.nvim_create_augroup('nvim_spellfile', { clear = true }),
-  pattern = '*',
+  group = vim.api.nvim_create_augroup('nvim.spellfile', {}),
   desc = 'Download missing spell files when setting spelllang',
-  callback = on_spellfile_missing,
+  callback = function(args)
+    require('nvim.spellfile').get(args.match)
+  end,
 })