feat(cli): Add support for nullable<T>

[amckinney]

This adds support for a new Fern definition keyword, nullable. With this, users can explicitly specify whether or not a type is allowed to be serialized as null, which is valid for both required and optional properties.

With this, users can specify APIs like the following:

types:
  User:
    properties:
      name: string
      email: nullable<string>
      createdAt: optional<string>
      updatedAt: optional<string>

service:
  auth: false
  base-path: /
  endpoints:
    createUser:
      method: POST
      path: /users
      request:
        name: CreateUserRequest
        body:
          properties:
            name: string
            email: nullable<string>
      response: User

A few things to note:

1. The nullable type is migrated to optional whenever the target generator doesn't support the latest IR.
2. A follow-up will introduce an opt-in OpenAPI setting that converts nullable OpenAPI properties to nullable.
   • By default, these properties are converted to optional.
3. In another follow-up the dynamic snippet generators will be updated to allow for explicit null values.

[github-actions]

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