refactor(vim.fs): deduplicate by using vim.fn #39782

Problem:
61e99217e6 replaced usages of `vim.fn`. This duplicates non-trivial
logic and may have introduced bugs like 38e38d1b40.

Later on, b02eeb6a72 graduated `fnamemodify` to `fast`, so avoiding it
in `vim.fs` is no longer necessary.

Solution:
Use `vim.fn` to deduplicate `vim.fs.dirname()` and `vim.fs.basename()`.

Note: the "nvim -l" test-runner switch from the original PR (#30483) is
already done by 9432e6c1e2 (#39676).

runtime/lua/vim/fs.lua

@@ -44,9 +44,7 @@ local uv = vim.uv
 
 local M = {}
 
--- Can't use `has('win32')` because the `nvim -ll` test runner doesn't support `vim.fn` yet.
-local sysname = uv.os_uname().sysname:lower()
-local iswin = not not (sysname:find('windows') or sysname:find('mingw'))
+local iswin = vim.fn.has('win32') == 1
 local os_sep = iswin and '\\' or '/'
 
 --- Iterate over all the parents of the given path (not expanded/resolved, the caller must do that).
@@ -90,27 +88,15 @@ end
 ---@since 10
 ---@generic T : string|nil
 ---@param file T Path
----@return T Parent directory of {file}
+---@return T # Parent directory of `file`
 function M.dirname(file)
   if file == nil then
     return nil
   end
   vim.validate('file', file, 'string')
+  local dir = vim.fn.fnamemodify(file, ':h')
   if iswin then
-    file = file:gsub(os_sep, '/') --[[@as string]]
-    if file:match('^%w:/?$') then
-      return file
-    end
-  end
-  if not file:match('/') then
-    return '.'
-  elseif file == '/' or file:match('^/[^/]+$') then
-    return '/'
-  end
-  ---@type string
-  local dir = file:match('/$') and file:sub(1, #file - 1) or file:match('^(/?.+)/')
-  if iswin and dir:match('^%w:$') then
-    return dir .. '/'
+    return (dir:gsub(os_sep, '/'))
   end
   return dir
 end
@@ -120,19 +106,17 @@ end
 ---@since 10
 ---@generic T : string|nil
 ---@param file T Path
----@return T Basename of {file}
+---@return T # Basename of `file`
 function M.basename(file)
   if file == nil then
     return nil
   end
   vim.validate('file', file, 'string')
+  local name = vim.fn.fnamemodify(file, ':t')
   if iswin then
-    file = file:gsub(os_sep, '/') --[[@as string]]
-    if file:match('^%w:/?$') then
-      return ''
-    end
+    return (name:gsub(os_sep, '/'))
   end
-  return file:match('/$') and '' or (file:match('[^/]*$'))
+  return name
 end
 
 --- Concatenates partial paths (one absolute or relative path followed by zero or more relative