follow-schema support for model files

### What happened?

Resolvers can be broken up into separate files that track schema filenames (but not directories, separate issue). This is done with
```
resolver:
layout: follow-schema
```
The `{name}` token can also be used in the filename that gets generated. This is also implemented for `exec`.

However, the modelgen plugin does not have support for this.

### What did you expect?

That model files could be generated by name following schema.

### Minimal graphql.schema and models to reproduce

### versions
- `go run github.com/99designs/gqlgen version`?
- `v0.17.48`
- `go version`?
- `go version go1.22.4 darwin/arm64`