Split runtime into multiple files

Runtime/rollup.config.js

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+import typescript from "@rollup/plugin-typescript";
+
+/** @type {import('rollup').RollupOptions} */
+const config = {
+    input: "src/index.ts",
+    output: [
+        {
+            file: "lib/index.mjs",
+            format: "esm",
+        },
+        {
+            dir: "lib",
+            format: "umd",
+            name: "JavaScriptKit",
+        },
+    ],
+    plugins: [typescript()],
+};
+
+export default config;

Runtime/src/closure-heap.ts

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+import { ExportedFunctions } from "./types";
+
+/// Memory lifetime of closures in Swift are managed by Swift side
+export class SwiftClosureDeallocator {
+    private functionRegistry: FinalizationRegistry<number>;
+
+    constructor(exports: ExportedFunctions) {
+        if (typeof FinalizationRegistry === "undefined") {
+            throw new Error(
+                "The Swift part of JavaScriptKit was configured to require " +
+                    "the availability of JavaScript WeakRefs. Please build " +
+                    "with `-Xswiftc -DJAVASCRIPTKIT_WITHOUT_WEAKREFS` to " +
+                    "disable features that use WeakRefs."
+            );
+        }
+
+        this.functionRegistry = new FinalizationRegistry((id) => {
+            exports.swjs_free_host_function(id);
+        });
+    }
+
+    track(func: Function, func_ref: number) {
+        this.functionRegistry.register(func, func_ref);
+    }
+}

Runtime/src/find-global.ts

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+interface GlobalVariable {}
+declare const global: GlobalVariable;
+
+export let globalVariable: any;
+if (typeof globalThis !== "undefined") {
+    globalVariable = globalThis;
+} else if (typeof window !== "undefined") {
+    globalVariable = window;
+} else if (typeof global !== "undefined") {
+    globalVariable = global;
+} else if (typeof self !== "undefined") {
+    globalVariable = self;
+}