fix: use export type for SwiftRuntimeThreadChannel re-export

SwiftRuntimeThreadChannel is a type alias, not a value. Re-exporting
it with `export { ... }` causes esbuild and rolldown (which run in
isolatedModules mode) to fail with "cannot re-export a type using
'export { ... }'" because the bundler cannot tell the export is
type-only at the module boundary.

Changing to `export type { SwiftRuntimeThreadChannel }` makes the
intent explicit and restores compatibility with esbuild/rolldown-based
build tools (Vite, etc.) without affecting tsc or any runtime
behaviour.

Reported in #477

Author: shivanshu877

Runtime/src/index.ts

@@ -20,7 +20,7 @@ import {
 } from "./itc.js";
 import { decodeObjectRefs } from "./js-value.js";
 import { JSObjectSpace } from "./object-heap.js";
-export { SwiftRuntimeThreadChannel };
+export type { SwiftRuntimeThreadChannel };
 
 export type SwiftRuntimeOptions = {
     /**