Issue with declare class fields

I'm not entirely sure that the problem is related to this addon specifically, but it is clearly related to the `*.gts.` files

I have a situation where I need to extend a component from an addon. The addon class has a field `interactive` and I need to check this field to be true before making any further decisions in the code.

I had this working piece of code in a `*.ts` file and decided to move it to `*.gts`.

```ts
import SomeAddonComponent from 'some-addon/components/some-component';
import layout from 'some-addon/templates/components/some-component';

class MyComponent extends SomeAddonComponent {
declare interactive: boolean;
someFunction() {
if (this.interactive)
...
super.someFunction(event);
}
}

export default setComponentTemplate(layout, MyComponent);
```

`declare interactive` is used because the linter complains about the absence of the `this.interactive` field, in order to actually declare the existence of the field forcibly

After changing the extension to `*.gts` the code stopped working correctly. As a result of debugging, I noticed that `this.interactive` contains `undefined` instead of a boolean value. Looking at the prototype of the object, I noticed that the class actually overrides the `interactive` field, although the `*.ts` files do not do this.

I perceived `declare` as part of typescript, so I can’t fully understand, am I using things for other purposes, or is there some kind of conflict with the `*.gts` files?

Start by reproducing the component inheritance in equivalent *.ts and *.gts files, then inspect the generated class prototype and the value of this.interactive. The issue is resolved when the difference is explained and the expected inherited-field behavior is either restored or clearly documented, with a regression test if the repository has a suitable test entry point.

Written by the indexing model from the issue text.