Support of string-based enums

I know this kind of a limitation of proto protocol, but would be good to have the ability to have string-based enums in TS.

enum EmploymentStatus {
  unemployed = 0;
  employed = 1;
  student = 2;
  retired = 3;
}

it intended to be in the TS:

enum EmploymentStatus = {
  unemployed = 'unemployed';
  employed = 'employed';
  student = 'student';
  retired = 'retired';
}

[thesayyn]

also, using oneof fileds could be an option.

message EmploymentStatus {
  oneof name {
      bool unemployed = 0;
      bool employed = 1;
      bool student = 2;
      bool retired = 3;
  }
}


```typescript
const status = new EmploymentStatus({unemployed: true});
status.name // "unemployed" | "employed" | "student" | "retired"

since one of fields can present one at a time, this would act like an enum and we wouldn’t have to worry about wire format or interoperability as this is a normal way of using the language.

[tomasweigenast]

@thesayyn Here it says the ENUM specification: https://developers.google.com/protocol-buffers/docs/proto3#json It must be a string, not an integer. If you use Protobuf with dart, it will serialize enums as strings, and this plugin will not understand that.