fix(vim.fs): dirname() returns "." on mingw/msys2 #30480

Problem:
`vim.fs.dirname([[C:\User\XXX\AppData\Local]])` returns "." on
mingw/msys2.

Solution:
- Check for "mingw" when deciding `iswin`.
- Use `has("win32")` where possible, it works in "fast" contexts since
  b02eeb6a72.

runtime/lua/vim/fs.lua

@@ -2,7 +2,9 @@ local uv = vim.uv
 
 local M = {}
 
-local iswin = uv.os_uname().sysname == 'Windows_NT'
+-- 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 os_sep = iswin and '\\' or '/'
 
 --- Iterate over all the parents of the given path.