meta: improve typing for unpack

Fixes #1353

Author: lewis6991

meta/template/basic.lua

@@ -339,3 +339,10 @@ function xpcall(f, msgh, arg1, ...) end
 ---@return T   ...
 ---@nodiscard
 function unpack(list, i, j) end
+
+---@version 5.1
+---@generic T1, T2, T3, T4, T5, T6, T7, T8, T9
+---@param list {[1]: T1, [2]: T2, [3]: T3, [4]: T4, [5]: T5, [6]: T6, [7]: T7, [8]: T8, [9]: T9 }
+---@return T1, T2, T3, T4, T5, T6, T7, T8, T9
+---@nodiscard
+function unpack(list) end