fix(28516): Abstract methods should not allow async modifier

[a-tarasyuk]

Fixes #28516

[sandersn]

Looks good to me.

This is a breaking change, so I'm marking it as such. However, I couldn't find any examples of its use on my hard drive, which has a lot of TS and JS on it. =)

[MikeMitterer]

And now??? How should we implement a concrete async function in an abstract class???

This seems not be possible anymore:

export abstract class Base {
    public abstract getName(): string;
    
    public async getAge(): Promise<number> {
        return await fetchAgeFromServer()
    }
}

class Real extends Base {
    getName(): string {
        return 'MyName';
    }
}