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>
(cherry picked from commit 902b689c4d)

runtime/doc/builtin.txt

@@ -12029,7 +12029,7 @@ winlayout([{tabnr}])                                               *winlayout()*
                   • {tabnr} (`integer?`)
 
                 Return: ~
-                  (`any[]`)
+                  (`vim.fn.winlayout.ret`)
 
 winline()                                                            *winline()*
 		The result is a Number, which is the screen line of the cursor

runtime/lua/vim/_meta/builtin_types.lua

@@ -297,3 +297,18 @@
 --- A list of dictionaries with information about
 --- undo blocks.
 --- @field entries vim.fn.undotree.entry[]
+
+--- @class vim.fn.winlayout.leaf
+--- @field [1] "leaf" Node type
+--- @field [2] integer winid
+
+--- @class vim.fn.winlayout.branch
+--- @field [1] "row" | "col" Node type
+--- @field [2] (vim.fn.winlayout.leaf|vim.fn.winlayout.branch)[] children
+
+--- @class vim.fn.winlayout.empty
+
+--- @alias vim.fn.winlayout.ret
+--- | vim.fn.winlayout.leaf
+--- | vim.fn.winlayout.branch
+--- | vim.fn.winlayout.empty

runtime/lua/vim/_meta/vimfn.lua

@@ -10929,7 +10929,7 @@ 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

src/nvim/eval.lua

@@ -13239,7 +13239,7 @@ M.funcs = {
     ]=],
     name = 'winlayout',
     params = { { 'tabnr', 'integer' } },
-    returns = 'any[]',
+    returns = 'vim.fn.winlayout.ret',
     signature = 'winlayout([{tabnr}])',
   },
   winline = {