[Vite] Use the same generated config pattern as with webpack

[Artur-]

There should be a vite.generated.ts which we always overwrite and then a vite.config.ts that is only generated if it does not exist.

[Artur-]

Maybe this would work

vite.generated.ts

/**
 * NOTICE: this is an auto-generated file
 *
 * This file has been generated by the `flow:prepare-frontend` maven goal.
 * This file will be overwritten on every run. Any custom changes should be made to vite.config.ts
 */

import { UserConfigExport } from 'vite';

...

export const vaadinConfig: UserConfigExport = {
   ...
};

vite.config.ts

import { defineConfig, UserConfigExport } from 'vite';
import { vaadinConfig } from './vite.generated';

// https://vitejs.dev/config/
const customConfig: UserConfigExport = {
};

export default defineConfig({ ...vaadinConfig, ...customConfig });

[Artur-]

There is actually a mergeConfig exported by Vite that we probably should use

[vaadin-bot]

This ticket/PR has been released with platform 22.0.0.beta2 and is also targeting the upcoming stable 22.0.0 version.