Use babel to support old browser versions up to firefox 52

svelte/babel.config.cjs

@@ -0,0 +1,15 @@
+module.exports = function (api) {
+	api.cache(true);
+	return {
+		presets: [
+			[
+				"@babel/env",
+				{
+					"targets": {
+						"firefox": "52"
+					}
+				}
+			]
+		]
+	};
+}

svelte/package.json

@@ -8,6 +8,9 @@
     "start": "sirv public"
   },
   "devDependencies": {
+    "@babel/core": "^7.22.20",
+    "@babel/preset-env": "^7.22.20",
+    "@rollup/plugin-babel": "^6.0.3",
     "@rollup/plugin-commonjs": "^25.0.0",
     "@rollup/plugin-node-resolve": "^15.2.1",
     "@rollup/plugin-terser": "^0.4.3",

svelte/rollup.config.js

@@ -3,6 +3,7 @@ import resolve, { nodeResolve } from '@rollup/plugin-node-resolve';
 import commonjs from '@rollup/plugin-commonjs';
 import livereload from 'rollup-plugin-livereload';
 import terser from '@rollup/plugin-terser';
+import babel from '@rollup/plugin-babel'
 
 const production = !process.env.ROLLUP_WATCH;
 
@@ -37,6 +38,11 @@ export default [
 			emitCss: false,
 		}),
 
+		babel({
+			extensions: [".js", ".html", ".svelte"],
+			babelHelpers: "bundled",
+		}),
+
 		// If you have external dependencies installed from
 		// npm, you'll most likely need these plugins. In
 		// some cases you'll need additional configuration -