Add support for custom package name in generated go file

[andrzejressel]

I would like to use generated file from within a library and I cannot have package main there.

[bahlo]

+1, this would unblock our workflow as well

[lirenjie95]

Thank you for providing this PR! I have a few suggestions:

1. Could you please add some tests to verify that the changes work correctly? For instance, it would be helpful to test edge cases such as invalid names, Go keywords, or names starting with a number. Additionally, would you mind checking the documentation and updating the support for custom package names if needed? If you want me to help with the validation functions, I’ll be happy to do so!
2. I noticed that your code ensures the default package name is "main". However, I'm a bit unclear about line 149 — it still returns "main" when the package name is empty. Could you help me understand the reasoning behind this? (I might be missing something here, so please feel free to correct me if I'm mistaken.)

Also cc @ihciah

[andrzejressel]

1. Then user provide some invalid package it's their problem. I don't think any filtering/checking is needed - Go compiler will return error anyway. I can create additional example example-go-library if needed.
2. The Default::default() is used everywhere - so the default value is empty string. I guess I can change the type to Option<String> and use main when the option is None.

[quinn]

i think this change is necessary to make this tool useful (very unlikely main will be used in anything other than an example) any chance this could be merged? thank you!

[lirenjie95]

@ihciah Can you help review this PR? Looks like it’s reasonable and effective.