[HTML5] Implement Godot <-> JavaScript interface.

doc/classes/JavaScriptObject.xml

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+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="JavaScriptObject" inherits="Reference" version="3.4">
+	<brief_description>
+		A wrapper class for native JavaScript objects.
+	</brief_description>
+	<description>
+		JavaScriptObject is used to interact with JavaScript objects retrieved or created via [method JavaScript.get_interface], [method JavaScript.create_object], or [method JavaScript.create_callback].
+		Example:
+		[codeblock]
+		extends Node
+
+		var _my_js_callback = JavaScript.create_callback(self, "myCallback") # This reference must be kept
+		var console = JavaScript.get_interface("console")
+
+		func _init():
+		    var buf = JavaScript.create_object("ArrayBuffer", 10) # new ArrayBuffer(10)
+		    print(buf) # prints [JavaScriptObject:OBJECT_ID]
+		    var uint8arr = JavaScript.create_object("Uint8Array", buf) # new Uint8Array(buf)
+		    uint8arr[1] = 255
+		    prints(uint8arr[1], uint8arr.byteLength) # prints 255 10
+		    console.log(uint8arr) # prints in browser console "Uint8Array(10) [ 0, 255, 0, 0, 0, 0, 0, 0, 0, 0 ]"
+
+		    # Equivalent of JavaScript: Array.from(uint8arr).forEach(myCallback)
+		    JavaScript.get_interface("Array").from(uint8arr).forEach(_my_js_callback)
+
+		func myCallback(args):
+		    # Will be called with the parameters passed to the "forEach" callback
+		    # [0, 0, [JavaScriptObject:1173]]
+		    # [255, 1, [JavaScriptObject:1173]]
+		    # ...
+		    # [0, 9, [JavaScriptObject:1180]]
+		    print(args)
+		[/codeblock]
+		Note: Only available in the "HTML5" platform.
+	</description>
+	<tutorials>
+	</tutorials>
+	<methods>
+	</methods>
+	<constants>
+	</constants>
+</class>