Break apart and improve the JS for rasterizing SVGs and downloading files (#786)

It now uses a blob URL instead of a data URL in the download process, which is cleaner and better performance.

frontend/src/wasm-communication/messages.ts

@@ -222,7 +222,7 @@ export class TriggerImport extends JsMessage {}
 export class TriggerPaste extends JsMessage {}
 
 export class TriggerRasterDownload extends JsMessage {
-	readonly document!: string;
+	readonly svg!: string;
 
 	readonly name!: string;