Custom output rollup configuration is overhidden by the default configuration

Describe the bug
In the svelte configuration file when we try to set the assetFileNames or chunkFileNames in output rollup, this is overhidden by the default configuration

To Reproduce
Set a assetFileNames in the rollup options and check the assets files.

Expected behavior
The configuration set in the svelte configuration file must be used.

Information about your SvelteKit Installation:
Next version: 103

Severity
Blocking our usage of SvelteKit: declare static asset files in the micro-frontend application

Additional context
This portion of code seems to be the origin of the issue: https://github.com/sveltejs/kit/blob/57efb032983a614462bae6b1d7496072b6aa8c96/packages/kit/src/core/build/index.js#L149

	rollupOptions: {
		...(user_config.build && user_config.build.rollupOptions),
		input,
		output: {
			entryFileNames: '[name]-[hash].js',
			chunkFileNames: 'chunks/[name]-[hash].js',
			assetFileNames: 'assets/[name]-[hash][extname]'
		},
		preserveEntrySignatures: 'strict'
	}
},

Start in packages/kit/src/core/build/index.js at the rollupOptions output configuration linked in the issue. Reproduce the problem by setting assetFileNames or chunkFileNames in the Svelte configuration and checking the generated assets. Done means those user-provided output settings are used instead of being overwritten by the defaults.