Support enumName without case conversion

[sladomic]

Hi and thanks for the awesome package!

We use ProtobufEnums in our project which we would like to import as contexts. They are always generated in the form of Example_Enum with an underline in between.

The problem right now is, that the enumName is changed to pascal case and therefore it regenerates the enum even though we set generate_enum to false.

It's about this line of code. Could we either remove it or have it as a config?

https://github.com/slang-i18n/slang/blob/266523faeac1d3ee4d6b72d759931456593430e6/slang/lib/builder/builder/raw_config_builder.dart#L121