add firefox extension template

externs/webextension_polyfill/clipboard/SetImageDataImageTypeEnum.hx

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+package webextension_polyfill.clipboard;
+
+/**
+	The type of imageData.
+**/
+typedef SetImageDataImageTypeEnum = String;

externs/webextension_polyfill/clipboard/Static.hx

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+package webextension_polyfill.clipboard;
+
+typedef Static = {
+	/**
+		Copy an image to the clipboard. The image is re-encoded before it is written to the clipboard. If the image is invalid,
+		the clipboard is not modified.
+	**/
+	function setImageData(imageData:js.lib.ArrayBuffer, imageType:SetImageDataImageTypeEnum):js.lib.Promise<ts.Undefined>;
+};