apollo client:codegen Option for different TypeScript enum output

Currently, the following GraphQL type
```
enum Color {
RED
BLUE
}
```

generates the following TypeScript type
```
export enum Color {
RED = "RED",
BLUE = "BLUE",
}
```

I would like to have the option to generate the following type instead:
```
export type Color = "RED" | "BLUE";
```

This definition provides the same safety, but is more efficient and works better with TypeScript structural typing.

Unrelated: consider generating `const enum` instead of `enum`.

The issue does not name files or tests. Start by locating the TypeScript GraphQL code-generation path that emits enum declarations and inspect how generator options are defined. Done means adding a supported option for string-union output, preserving the existing enum behavior, and covering both forms with tests.

Written by the indexing model from the issue text.