union(protogen): check fields' length to avoid prefix conflict of subsequent field name

[wenchy]

For example, the field TargetProgress has the same prefix Target with the union prefix in previous columns.

ID	TargetType	TargetField1	TargetField2	TargetField3	TargetProgress
map<int32, Task>	{.Target}enum<.Target.Type>	union	union	union	int32
ID	Target's type	Target's field1	Target's field2	Target's field3	Progress
1	PVP	1	10	Apple,Orange,Banana	3

But currently, the confgen works fine even if column names are not in order. So this is under discusion whether there is need to resolve it?