lua: fix generated code for adding table

[serprex]

Example with issue:

table Test { x: int; }
table TestContainer { a: Test; b: Test; }

Generated code has TestContainer.AddA calling PrependStructSlot. Looking at the sample_binary.lua it's using explicit builder:PrependUOffsetTRelative to setup weapon vector. Update generated code to use PrependUOffsetTRelativeSlot