Source maps not being generated when using `outExtension { js: ".jsx" }`

## What's the problem?

When using `outExtension: () => ({ js: ".jsx" })` Tsup doesn not generate source maps. The build works but the missing source maps lead to a very bad debugging experience for us. Would be great to have these for jsx outputs as well.

## Longer Story

We're using SolidJS and need to preserve the JSX in library code of a big monorepo. The libs are built with tsup while the application is built with vite. The application uses `vite-plugin-solid` which expects all library code expose the unbuilt jsx and then builds the JSX in files with `.jsx` extensions in one batch. The build works but the missing source maps lead to a very bad debugging experience for us.

## Reproduction

```tsx
// tsup.config.ts
import { defineConfig } from "tsup";

export default defineConfig({
format: "esm",
entry: ["src/index.tsx"],
outDir: "out",
sourcemap: true,
esbuildOptions(es_options) {
es_options.jsx = "preserve";
},
outExtension: () => ({ js: ".jsx" }),
});
```

```tsx
// src/index.tsx
export function Component() {
return

;
}
```

```json
// package.json
{
"name": "lib",
"type": "module",
"exports": "./out/index.jsx",
"scripts": {
"build": "tsup"
},
"dependencies": {
"solid-js": "~1.9.4"
},
"devDependencies": {
"typescript": "~5.2.2",
"tsup": "~8.3.6"
}
}

```

Now run tsup

### Expected behavior

The `out` folder should contain `index.jsx` and `index.jsx.map`.

### Actual behavior

The `out` folder only contains `index.jsx` and no `index.jsx.map`.

### Narrowing it down

When commenting out the `outExtension` option in the `tsup.config.ts` the maps start appearing.

## Additional info

It does not seem to be an upstream esbuild problem. I ran esbuild with the same config that tsup calls it with and the source maps were there. They aren't when run through tsup though. I think the issue might lie in the this code that's responsible for writing to disc https://github.com/egoist/tsup/blob/0328fd64a771adb0789968775c22b4317683fafe/src/plugin.ts#L131-L225

Start with the reproduction in tsup.config.ts and src/index.tsx, run tsup with sourcemap enabled and outExtension set to .jsx, then inspect src/plugin.ts around lines 131-225. Done means the out directory contains both index.jsx and index.jsx.map, while the existing build output remains functional.

Written by the indexing model from the issue text.