[js] Add externs for the `BeforeInstallPromptEvent` class (#10399)

* [js] Add externs for the `BeforeInstallPromptEvent` class

* Fix a typo

std/js/html/BeforeInstallPromptEvent.hx

@@ -0,0 +1,33 @@
+package js.html;
+
+import js.lib.Promise;
+
+/**
+	The `BeforeInstallPromptEvent` is fired at the `Window.onbeforeinstallprompt` handler,
+	before a user is prompted to install a web site to a home screen.
+
+	@see https://developer.mozilla.org/en-US/docs/Web/API/BeforeInstallPromptEvent
+**/
+extern class BeforeInstallPromptEvent extends Event {
+
+	/** The platforms on which this event was dispatched. **/
+	final platforms: Array<String>;
+
+	/** The user's choice to the install prompt. **/
+	final userChoice: Promise<BeforeInstallPromptUserChoice>;
+
+	/** Creates a new `BeforeInstallPromptEvent`. **/
+	function new();
+
+	/** Shows the install prompt. **/
+	function prompt(): Promise<Dynamic>;
+}
+
+typedef BeforeInstallPromptUserChoice = {
+	final outcome: BeforeInstallPromptUserChoiceOutcome;
+}
+
+enum abstract BeforeInstallPromptUserChoiceOutcome(String) {
+	var Accepted = "accepted";
+	var Dismissed = "dismissed";
+}