fix(runtime)!: move "tohtml" to pack/dist/opt/ #34557

Problem:
The "tohtml" plugin is loaded by default.

Solution:
- Move it to `pack/dist/opt/nvim.tohtml/`, it is an "opt-in" plugin now.
- Document guidelines.
- Also revert the `plugin/` locations of `spellfile.lua` and `net.lua`.
  That idea was not worth the trouble, it will be too much re-education
  for too little gain.

runtime/plugin/spellfile.lua

@@ -0,0 +1,12 @@
+if vim.g.loaded_spellfile_plugin ~= nil then
+  return
+end
+vim.g.loaded_spellfile_plugin = true
+
+vim.api.nvim_create_autocmd('SpellFileMissing', {
+  group = vim.api.nvim_create_augroup('nvim.spellfile', {}),
+  desc = 'Download missing spell files when setting spelllang',
+  callback = function(args)
+    require('nvim.spellfile').get(args.match)
+  end,
+})