feat: allow function passed to defaulttable to take an argument (#22839)

Pass the value of the key being accessed to the create function, to
allow users to dynamically generate default values.

runtime/lua/vim/shared.lua

@@ -796,13 +796,15 @@ end
 --- a.b.c = 1
 --- </pre>
 ---
----@param create function|nil The function called to create a missing value.
+---@param create function?(key:any):any The function called to create a missing value.
 ---@return table Empty table with metamethod
 function vim.defaulttable(create)
-  create = create or vim.defaulttable
+  create = create or function(_)
+    return vim.defaulttable()
+  end
   return setmetatable({}, {
     __index = function(tbl, key)
-      rawset(tbl, key, create())
+      rawset(tbl, key, create(key))
       return rawget(tbl, key)
     end,
   })