feat(runtime): replace netrw with a very small script #39723

Problem:
`:edit <dir>` and `nvim <dir>` currently rely on netrw to show local directory
contents.

Solution:
- Provide `filetype=directory`.
- Introduce dir.lua, a small plugin that provides directory listing, opening
  items, parent navigation, and refresh.
- `netrw` remains available for `:Explore`, remote paths, archives, and file
  operations. To continue 

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

runtime/plugin/dir.lua

@@ -0,0 +1,67 @@
+if vim.g.loaded_nvim_dir_plugin ~= nil then
+  return
+end
+vim.g.loaded_nvim_dir_plugin = true
+
+local api = vim.api
+local nvim_on = require('vim._core.util').nvim_on
+
+vim.keymap.set('n', '<Plug>(nvim-dir-open)', function()
+  require('nvim.dir')._open_entry()
+end, { silent = true, desc = 'Open directory entry' })
+
+vim.keymap.set('n', '<Plug>(nvim-dir-up)', function()
+  require('nvim.dir')._open_parent()
+end, { silent = true, desc = 'Open parent directory' })
+
+vim.keymap.set('n', '<Plug>(nvim-dir-reload)', function()
+  require('nvim.dir')._reload()
+end, { silent = true, desc = 'Reload directory' })
+
+---@param buf integer
+---@param path string
+---@return boolean
+local function should_open(buf, path)
+  if path == '' then
+    return false
+  end
+  if vim.bo[buf].buftype ~= '' and vim.b[buf].nvim_dir == nil then
+    return false
+  end
+  if vim.bo[buf].filetype == 'netrw' or vim.b[buf].netrw_curdir ~= nil then
+    return false
+  end
+  return vim.fn.isdirectory(path) == 1
+end
+
+local group = api.nvim_create_augroup('FileExplorer', { clear = true })
+-- Latch on our own VimEnter, not v:vim_did_enter (set just before VimEnter
+-- autocmds), so an earlier VimEnter autocmd's BufEnter can't preempt startup.
+local vimentered = vim.v.vim_did_enter == 1
+
+nvim_on('BufEnter', group, {
+  pattern = '*',
+  desc = 'Open local directories',
+  nested = true,
+}, function(ev)
+  if vimentered and should_open(ev.buf, ev.file) then
+    require('nvim.dir').try_open(ev.buf, ev.file)
+  end
+end)
+
+nvim_on('VimEnter', group, {
+  pattern = '*',
+  desc = 'Open startup local directories',
+  nested = true,
+}, function()
+  vimentered = true
+  for _, win in ipairs(api.nvim_tabpage_list_wins(0)) do
+    if api.nvim_win_is_valid(win) then
+      local buf = api.nvim_win_get_buf(win)
+      if should_open(buf, api.nvim_buf_get_name(buf)) then
+        require('nvim.dir').handle_startup_dirs()
+        return
+      end
+    end
+  end
+end)