[C#] Flatc enhancement: Expose table offsets and lengths by name

I would be great if the internal offsets would also be exposed in the generated code as named constants and that these are also actually used by the generated code. This makes things a lot more readable and gives devs the option to also reuse these consts instead of magic numbers:

current
```
public float3? Vertices(int j) { int o = __p.__offset(4); return o != 0 ? (float3?) (new float3()).__assign(__p.__vector(o) + j * 12, __p.bb) : null; }
public int VerticesLength { get { int o = __p.__offset(4); return o != 0 ? __p.__vector_len(o) : 0; } }
public int Tris(int j) { int o = __p.__offset(6); return o != 0 ? __p.bb.GetInt(__p.__vector(o) + j * 4) : (int) 0; }
public int TrisLength { get { int o = __p.__offset(6); return o != 0 ? __p.__vector_len(o) : 0; } }
```

proposed
```
public class TrimeshTable
{
public const int VerticesOffset = 4;
public const int VerticesLen = 12;
public const int TrisOffset = 6;
public const int TrisLen = 4;
}
public float3? Vertices(int j) { int o = __p.__offset(TrimeshTable.VerticesOffset); return o != 0 ? (float3?) (new float3()).__assign(__p.__vector(o) + j * TrimeshTable.VerticesLen, __p.bb) : null; }
public int VerticesLength { get { int o = __p.__offset(TrimeshTable.VerticesOffset); return o != 0 ? __p.__vector_len(o) : 0; } }
public int Tris(int j) { int o = __p.__offset(TrimeshTable.TrisOffset); return o != 0 ? __p.bb.GetInt(__p.__vector(o) + j * TrimeshTable.TrisLen) : (int) 0; }
public int TrisLength { get { int o = __p.__offset(TrimeshTable.TrisOffset); return o != 0 ? __p.__vector_len(o) : 0; } }
```