Double initializing a `readonly` field should not be allowed

Nobody has claimed this yet.

### 🔎 Search Terms

readonly double twice initializer

### 🕗 Version & Regression Information

- This is the behavior in every version I tried, and I reviewed the FAQ for entries about initializers?

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYGwhgzhAEBiD29oG8BQ0PQE4FMwBN4A7EAT2gCMwsAuaCAFywEsiBzaAXmgHJXmGzMCGYAvHFmgBGHgG5U6TMGKMsAV2AN4WABQBKFIsyYGAC2YQAdFUnc+RAUJHjJAJjlHoAX1Q-Uyogh4EBxLEHg2HSIcAHc4RH1raj1ZIA

### 💻 Code

```ts
class Foo {
readonly bar: string = 'initializer 1';

constructor() {
this.bar = 'initializer 2';
}
}

console.log(new Foo().bar);
```

### 🙁 Actual behavior

This compiles successfully despite overwriting a `readonly` field. This breaks expectations around `readonly`. If `Foo` happens to be a very large class and I only look at `readonly bar: string = 'initializer 1';` in isolation, I would expect any other line which reads `bar` to see `initializer 1`. However this is not a safe assumption because I need to check in the constructor to ensure it doesn't overwrite my `readonly` field.

### 🙂 Expected behavior

I expected a compiler error. TS should only allow _initializing_ `readonly` fields in a constructor, not overwriting a field which has already been initialized.

### Additional information about the issue

I get that we need `readonly` fields to be assignable in the constructor, but it feels like a bug to me that this is allowed when the field has already been initialized prior to the constructor. `readonly` should require a field initializer _xor_ a constructor initializer. Having both is invalid IMHO.

You can make the same argument that double initializing in the constructor should be invalid too:

```typescript
class Foo {
readonly bar: string;

constructor() {
this.bar = 'initializer 1';
this.bar = 'initializer 2'; // Should maybe error? Doesn't today.
}
}
```

But I understand that control flow analysis gets a lot more complicated in the constructor WRT `readonly` and might be considered a different feature with different priority.

Start by running the linked TypeScript Playground examples and compare the field-initializer case with the constructor-only double-assignment case. The change is done when the first example produces a compiler error for assigning to the already initialized readonly field, with the requested behavior covered by a regression test.

Written by the indexing model from the issue text.