union: support union custom variable name

[wenchy]

Refer https://tableauio.github.io/docs/excel/struct/#custom-named-struct

[Kybxd]

This does not make sense for now. Unions are all predefined structs, and the field name of predefined struct is not decided by its custom name. We just trim the suffix of its first field and use the rest part as it's field name.

[wenchy]

Mabye you misunderstand the problem.

Problem: For horizontal two union fields of same union type, how to resolve it? Solution: Just like https://tableauio.github.io/docs/excel/struct/#custom-named-struct, if custom name explicitly set, then the struct name prefix should be changed to the custom variable name but not the struct (or union) type name.