Add typescript support for structs

[dexsper]

• Allow [GenerateTypeScript] on struct declarations (managed and unmanaged)
• Managed structs use object-header wire format with non-nullable TS signatures; deserializeCore throws on null header instead of returning null
• Unmanaged structs use raw memory blit: field offsets and padding are computed at generation time and emitted as writeZeros/skipBytes calls
• Struct members in other types generate non-nullable TypeScript types
• Add MEMPACK043 diagnostic for Nullable members (use struct directly)
• Add writeZeros/skipBytes helpers to MemoryPackWriter/MemoryPackReader TS runtime
• Add BoundingBox (struct-in-struct) and GameObject (class + mixed structs) to sandbox
• Add comprehensive testStructs covering all struct combinations end-to-end