Support for annotating deprecated fields

[dpup]

From grpc-ecosystem#39

If a field is deprecated in the proto definitions, such as

message MyMessage {
  string field_a = 1;
  string field_b = 2 [deprecated = true];
}

Would it be possible to add a deprecated annotation to the generated types:

export type MyMessage = {
  fieldA?: string
  /**
   * @deprecated
   */
  fieldB?: string[]
}

If it did, the typescript compiler picks this up giving a warning.

This is useful when deprecating an old field (and often moving to a new one) in a large codebase as it gives a visual indication where the old one is used.