Fix Lua table field builder slot generation

[steadytao]

Summary

Make the Lua generator use PrependUOffsetTRelativeSlot for table fields instead of PrependStructSlot.

Problem

The Lua generator treated both structs and tables as Struct when selecting the builder slot method for table fields.

For a schema like:

table Customer {
  name:string;
  age:int;
}

table Sale {
  amount:int;
  customer:Customer;
}

it generated:

function Sale.AddCustomer(builder, customer)
  builder:PrependStructSlot(1, customer, 0)
end

but customer is a table offset, not an inline struct. This causes packing to fail with:
Tried to write a Struct at an Offset that is different from the current Offset of the Builder.

Fix

• only use Struct builder methods for actual struct fields
• use UOffsetTRelative for table fields
• add a focused Lua regression schema and test that packs a nested table successfully

Testing

• rebuilt flatc
• generated Lua for the nested Customer / Sale schema
• verified the generated Sale.AddCustomer now uses PrependUOffsetTRelativeSlot
• luatest.lua: 8 / 8 tests passed

Fixes #8611