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.

test/functional/lua/filetype_spec.lua

@@ -73,6 +73,15 @@ describe('vim.filetype', function()
     )
   end)
 
+  it('detects directory filenames', function()
+    eq(
+      'directory',
+      exec_lua(function()
+        return vim.filetype.match({ filename = '/tmp/example/' })
+      end)
+    )
+  end)
+
   it('works without defined g:ft_ignore_pat', function()
     local match_opts = { filename = 'unknown-ft', buf = api.nvim_create_buf(false, true) }
     eq(