docs(lua): typing for vim.fn.winlayout #33817

Problem:
`any[]` means nothing, and the return value is not the same as what's
documented in the comment (eg, Lua returns `{ "row", { { "leaf", 1000 },
{ "leaf", 1001 } } }`, not `{ "row", { "leaf", 1000, "leaf", 1001 } }`)

Solution:
Create two classes (vim.fn.winlayout.leaf and vim.fn.winlayout.branch)
and one alias that links the two together.

Also: Due to LuaLS limitations, there is an empty class,
vim.fn.winlayout.empty

Signed-Off-By: VoxelPrismatic <voxelprismatic@pm.me>

runtime/lua/vim/_meta/vimfn.lua

@@ -11002,7 +11002,7 @@ function vim.fn.winheight(nr) end
 --- <
 ---
 --- @param tabnr? integer
---- @return any[]
+--- @return vim.fn.winlayout.ret
 function vim.fn.winlayout(tabnr) end
 
 --- The result is a Number, which is the screen line of the cursor