[css-animationworklet] Move interface idls close to definitions for easier readability.

[flackr]

The Animation Worklet section introduces the CSS.animationWorklet interface and the AnimationWorkletGlobalScope, however the idl for paintWorklet follows the introduction of the global scope, and the idl for AnimationWorkletGlobalScope is in a later section.

I think for readability, similar to Paint Worklet the idl for each should follow their introduction in the Animation Worklet section.

i.e.

Animation Worklet {#animation-worklet-desc}
===========================================
<dfn>Animation Worklet</dfn> is a {{Worklet}} responsible for all classes related to custom
animations. The worklet can be accessed via {{animationWorklet}} attribute.

The {{animationWorklet}}'s <a>worklet global scope type</a> is {{AnimationWorkletGlobalScope}}.

<idl for animationWorklet>

{{AnimationWorkletGlobalScope}} represents the global execution context of {{animationWorklet}}.

<idl for AnimationWorkletGlobalScope>