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/doc/lua.txt

@@ -1635,8 +1635,8 @@ defaulttable({create})                                    *vim.defaulttable()*
 <
 
     Parameters: ~
-      • {create}  (function|nil) The function called to create a missing
-                  value.
+      • {create}  function?(key:any):any The function called to create a
+                  missing value.
 
     Return: ~
         (table) Empty table with metamethod

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,
   })

test/functional/lua/vim_spec.lua

@@ -2915,6 +2915,15 @@ describe('lua stdlib', function()
         return a
       ]])
     end)
+
+    it('accepts the key name', function()
+      eq({ b = 'b', c = 'c' }, exec_lua [[
+        local a = vim.defaulttable(function(k) return k end)
+        local _ = a.b
+        local _ = a.c
+        return a
+      ]])
+    end)
   end)
 
   it('vim.lua_omnifunc', function()