`ember/no-empty-glimmer-component-classes` probably shouldn't activate on `declare class`

[vlascik]

If you try to define ambient types for an addon, ember/no-empty-glimmer-component-classes probably shouldn't activate on declare class:

export declare class PowerSelectMultiple<T> extends Component<PowerSelectMultipleSignature<T>> {}

This is useful when doing e.g. ambient Glint component types.