Request to support snake case as a value form

### Is your feature request related to a problem? Please describe.

I'm working on a template for an ASP.NET gRPC project. The template includes ".proto" files with package names using snake_case, such as `package my_greeter`. I would like to be able to replace this with another symbol when generating a project using `dotnet new`.

I can see that [Value Forms](https://github.com/dotnet/templating/wiki/Value-Forms) might be able to help with my use case, but it does not currently support snake case. Please consider my request to add snake case as a value form.

### Describe the solution you'd like.

Add a new [Value Form](https://github.com/dotnet/templating/wiki/Value-Forms) to support snake_case, for instance:

`snakeCase` - Converts the value to snake case using the casing rules of the invariant culture.

```json
"forms": {
"snake": {
"identifier": "snakeCase"
}
}
```

I believe this could be implemented in a similar way to `kebabCase` as per [KebabCaseValueFormFactory](https://github.com/dotnet/templating/blob/main/src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/ValueForms/KebabCaseValueFormFactory.cs).

### Additional context

_No response_