Introduction of the AudioDestinationNode module

AudioDestinationNode is a subclass of AudioNode with no constructor and
no extra method. The only effective difference is the `maxChannelCount`
instance property, which is already covered by the
`audioDestinationNode` type in the `WebAudioAPI` module.

This PR allows for `AudioContext`'s `destination` property to be used
like an `AudioNode`, especially being connected to by other nodes since
this is the only way to output audio to speakers.

For more information, check
[MDN](https://developer.mozilla.org/en-US/docs/Web/API/AudioDestinationNode)

Author: julien-deoux

src/WebAudioAPI/AudioDestinationNode.js

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+open WebAudioAPI
+
+include AudioNode.Impl({
+  type t = audioDestinationNode
+})

src/WebAudioAPI/AudioDestinationNode.res

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+open WebAudioAPI
+
+let ctx = AudioContext.make()
+
+let destinationNode = ctx.destination->AudioDestinationNode.asAudioNode
+let context = AudioContext.asBaseAudioContext(ctx)
+
+let osc = OscillatorNode.make(
+  ~context,
+  ~options={
+    type_: Sine,
+    frequency: 440.0,
+  },
+)
+let gain = GainNode.make(
+  ~context,
+  ~options={
+    gain: 0.3,
+  },
+)
+let _ = gain->GainNode.connect(~destinationNode)
+let _ =
+  osc
+  ->OscillatorNode.asAudioScheduledSourceNode
+  ->AudioScheduledSourceNode.connect(~destinationNode=gain->GainNode.asAudioNode)
+
+osc->OscillatorNode.start