target: es5 outputs import statement

I have two files: index.ts and pixel.ts

Index has a bunch of functions and two classes

Pixel imports one of the classes and a function from index and instantiates the class, then passes it to the function.

I have this tsup config

```
/* eslint-disable no-var */
import { defineConfig, Options } from "tsup"
import pkg from './package.json'

var commonConfig: Options = {
clean: true,
splitting: false,
sourcemap: true,
env: {
__VERSION__: JSON.stringify(`${pkg.version}.${Date.now()}`),
__TARGET__: JSON.stringify("node"),
},
target: "node14",
outDir: "dist",
}

export default defineConfig([
{
entry: [
"src/pixel.ts"
],
...commonConfig,
format: ["iife"],
target: "es5",
env: {
...commonConfig.env,
__TARGET__: JSON.stringify("web"),
}
},
// {
// entry: [
// "src/index.ts",
// "src/types.ts",
// "src/examples.ts",
// ],
// ...commonConfig,
// format: ["esm", "cjs"],
// dts: true,
// },
])
```

...trying to cross compile esm modules, cjs modules for node14 and es5 for the web.

However, the output looks like this for the es5 build:

```
CLI Building entry: src/pixel.ts
CLI Using tsconfig: tsconfig.json
CLI tsup v6.2.2
CLI Using tsup config: /sdk/tsup.config.ts
CLI Target: es5
CLI Cleaning output folder
IIFE Build start
IIFE dist/pixel.global.js 47.72 KB
IIFE dist/pixel.global.js.map 40.93 KB
IIFE ⚡️ Build success in 62ms
```

Which then crashes with `import declarations may only appear at top level of a module` but obviously I want this pixel to work in very old browsers.

I call tsup like this:

tsup

and it reads the config file above in full.

Latest versions.