Describe the bug

When testing SvelteKit code, various parts can depend on whether running in the browser like so:

if (browser) window.addEventListener(...)

where browser is defined using esm-env, which does it using conditional exports in package.json:

"exports": {
	...
	"./browser": {
		"browser": "./true.js",
		"development": "./false.js",
		"production": "./false.js",
		"default": "./browser-fallback.js"
	},
	...
}

Now, development is set, so unless browser is also set, it will always be false. I know you can set export conditions in Vite, but I haven't seen anything similar in Vitest. I'd expect it to be set when using a browser environment, probably using an option or similar.

Reproduction

Stackblitz: https://stackblitz.com/edit/sveltejs-kit-template-default-muke8nsy
Steps: Just wait for install and run vitest in console

Workaround

I found this way to mock it, it works for me but I believe the property is also used by SvelteKit's vite plugin, which won't see this change, so there's some edge cases in which it might not work:

vi.mock(import("esm-env"), async (importOriginal): Promise<typeof import("esm-env")> => {
    const module = await importOriginal()
    if (typeof window == "object")
        return {
            ...module,
            BROWSER: true,
        }
    else return module
})

Interestingly you cannot wrap the entire call in an if (typeof window == "object") as the mock somehow leaks into tests for which the check fails, this way it re-mocks it and removes the property for those.