fix(filetype): ensure directory bufname ends w/ slash sep #40552

Problem:
`vim.filetype.match()` needs a cheap way to recognize directory buffers
without doing filesystem stat work.

Solution:
Ensure full buffer names for directories end in a trailing slash. Now
directory buffers can proceed through the normal 'filetype' path.

Note side-effects: session and ShaDa buffer-list restore behavior must
be compatible, so those + corresponding tests must be updated.

runtime/lua/nvim/dir.lua

@@ -158,8 +158,7 @@ end
 
 ---@param buf integer
 local function open_parent(buf)
-  local path = api.nvim_buf_get_name(buf)
-  -- TODO(barrettruth): simplify after #40552
+  local path = fs.normalize(api.nvim_buf_get_name(buf))
   local name = encode_name(fs.basename(path)) .. (vim.fn.isdirectory(path) == 1 and '/' or '')
   navigate(fs.dirname(path))
   vim.fn.search([[\C\m^\V]] .. vim.fn.escape(name, [[\]]) .. [[\m$]], 'cw')