feat(plugins): nvim.difftool can compare directories #35448

Problem:
Built-in diff mode (nvim -d) does not support directory diffing
as required by git difftool -d. This makes it difficult to compare
entire directories, detect renames, and navigate changes efficiently.

Solution:
Add a DiffTool plugin and command that enables side-by-side diffing of
files and directories in Neovim. The plugin supports rename detection,
highlights changes in the quickfix list, and provides a user command for
easy invocation. This allows proper integration with git difftool -d for
directory comparison.

Example git config:

```ini
[diff]
    tool = nvim_difftool

[difftool "nvim_difftool"]
    cmd = nvim -c "packadd nvim.difftool" -c "DiffTool $LOCAL $REMOTE"
```

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>
Co-authored-by: Phạm Bình An <111893501+brianhuster@users.noreply.github.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

runtime/lua/vim/_core/util.lua

@@ -0,0 +1,36 @@
+local M = {}
+
+--- Edit a file in a specific window
+--- @param winnr number
+--- @param file string
+--- @return number buffer number of the edited buffer
+M.edit_in = function(winnr, file)
+  return vim.api.nvim_win_call(winnr, function()
+    local current = vim.fs.abspath(vim.api.nvim_buf_get_name(vim.api.nvim_win_get_buf(winnr)))
+
+    -- Check if the current buffer is already the target file
+    if current == (file and vim.fs.abspath(file) or '') then
+      return vim.api.nvim_get_current_buf()
+    end
+
+    -- Read the file into the buffer
+    vim.cmd.edit(vim.fn.fnameescape(file))
+    return vim.api.nvim_get_current_buf()
+  end)
+end
+
+--- Read a chunk of data from a file
+--- @param file string
+--- @param size number
+--- @return string? chunk or nil on error
+function M.read_chunk(file, size)
+  local fd = io.open(file, 'rb')
+  if not fd then
+    return nil
+  end
+  local chunk = fd:read(size)
+  fd:close()
+  return tostring(chunk)
+end
+
+return M