Allow `readonly` with `accessor`

🔍 Search Terms

accessor
decorator
readonly

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Add readonly as a modifier for accessor fields.

📃 Motivating Example

Suppose you have an injection framework with @provides, @consumes, and @inject. Consider the example

class A {
  @provides(Number)
  readonly value: number;

  @inject([Number])
  accessor b = new B();

  constructor(value: number) {
    this.value = value;
  }
}

class B {
  @consumes(Number)
  readonly value: number;
}

Note because @provides is a field decorator, it cannot know when value is assigned unless it's in the initializer. As a consequence, b will not have value injected since @provides doesn't know when to reinject. This is fixed if you do

class A {
  @provides(Number)
  readonly accessor value: number;

  @inject([Number])
  accessor b = new B();

  constructor(value: number) {
    this.value = value;
  }
}

class B {
  @consumes(Number)
  readonly value: number;
}

💻 Use Cases

1. What do you want to use this for?
   Accessor declarations that shouldn't be modified after being used in the constructor.
2. What shortcomings exist with current approaches?
   You can only mark a field with @readonly using JSDoc and just hope that other developers don't touch the field.
3. What workarounds are you using in the meantime?
   @readonly using JSDoc

En el issue no se identifican archivos de código fuente, pruebas ni puntos de entrada de la implementación. Empieza rastreando cómo se analizan y comprueban las declaraciones de accessors y los modificadores readonly; después, añade cobertura que demuestre que las declaraciones de accessors readonly se aceptan y no se les puede reasignar un valor.