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.

src/nvim/eval.lua

@@ -823,7 +823,9 @@ M.funcs = {
     desc = [=[
       The result is the name of a buffer.  Mostly as it is displayed
       by the `:ls` command, but not using special names such as
-      "[No Name]".
+      "[No Name]".  If the buffer represents a directory, the name
+      ends with a path separator, unless it was changed by |:file| or
+      |nvim_buf_set_name()|.
       If {buf} is omitted the current buffer is used.
       If {buf} is a Number, that buffer number's name is given.
       Number zero is the alternate buffer for the current window.