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/api/buffer.c

@@ -942,6 +942,9 @@ void nvim_buf_del_var(Buffer buf, String name, Error *err)
 
 /// Gets the full/absolute filepath of the buffer, or the buffer name for non-file buffers.
 ///
+/// If the buffer represents a directory, the name ends with a path separator,
+/// unless it was changed by |:file| or |nvim_buf_set_name()|.
+///
 /// @param buf     Buffer id, or 0 for current buffer
 /// @param[out] err   Error details, if any
 /// @return Buffer name