feat(bundler): expose reactFastRefresh option in Bun.build API (#25731)

Fixes #25716

Adds support for a `reactFastRefresh: boolean` option in the `Bun.build`
JavaScript API, matching the existing `--react-fast-refresh` CLI flag.

```ts
const result = await Bun.build({
    reactFastRefresh: true,
    entrypoints: ["src/App.tsx"],
});
```

When enabled, the bundler adds React Fast Refresh transform code
(`$RefreshReg$`, `$RefreshSig$`) to the output.

packages/bun-types/bun.d.ts

@@ -1942,6 +1942,16 @@ declare module "bun" {
       development?: boolean;
     };
 
+    /**
+     * Enable React Fast Refresh transform.
+     *
+     * This adds the necessary code transformations for React Fast Refresh (hot module
+     * replacement for React components), but does not emit hot-module code itself.
+     *
+     * @default false
+     */
+    reactFastRefresh?: boolean;
+
     outdir?: string;
   }