[HTML5] Libraries refactor for linting.

Initial work to make liniting easier.

This includes:
- Rename http_request.js to library_godot_http_request.js.
- Rename externs.js to engine.externs.js.
- New library_godot_runtime.js (GodotRuntime) wraps around emscripten
  functions.
- Refactor of XMLHttpRequest handler in engine/preloader.js.
- Few fixes to bugs spotted by early stage linting.

platform/javascript/js/engine/utils.js

@@ -0,0 +1,57 @@
+var Utils = { // eslint-disable-line no-unused-vars
+
+	createLocateRewrite: function(execName) {
+		function rw(path) {
+			if (path.endsWith('.worker.js')) {
+				return execName + '.worker.js';
+			} else if (path.endsWith('.audio.worklet.js')) {
+				return execName + '.audio.worklet.js';
+			} else if (path.endsWith('.js')) {
+				return execName + '.js';
+			} else if (path.endsWith('.wasm')) {
+				return execName + '.wasm';
+			}
+			return path;
+		}
+		return rw;
+	},
+
+	createInstantiatePromise: function(wasmLoader) {
+		let loader = wasmLoader;
+		function instantiateWasm(imports, onSuccess) {
+			loader.then(function(xhr) {
+				WebAssembly.instantiate(xhr.response, imports).then(function(result) {
+					onSuccess(result['instance'], result['module']);
+				});
+			});
+			loader = null;
+			return {};
+		};
+
+		return instantiateWasm;
+	},
+
+	findCanvas: function() {
+		var nodes = document.getElementsByTagName('canvas');
+		if (nodes.length && nodes[0] instanceof HTMLCanvasElement) {
+			return nodes[0];
+		}
+		return null;
+	},
+
+	isWebGLAvailable: function(majorVersion = 1) {
+
+		var testContext = false;
+		try {
+			var testCanvas = document.createElement('canvas');
+			if (majorVersion === 1) {
+				testContext = testCanvas.getContext('webgl') || testCanvas.getContext('experimental-webgl');
+			} else if (majorVersion === 2) {
+				testContext = testCanvas.getContext('webgl2') || testCanvas.getContext('experimental-webgl2');
+			}
+		} catch (e) {
+			// Not available
+		}
+		return !!testContext;
+	}
+};