feat: export defaultAllowedOrigins for user-land config and 3rd party plugins

[timacdonald]

Description

It is not currently possible to widen the shipped allowed origins. A user or plugin must completely replace them defaults. The current value could be copy and pasted from core, but could then get out of sync.

This PR allows both user-land applications and 3rd-party plugins to augment the default allowed origins that ship with Vite without worrying about drift.

import { defineConfig, defaultAllowedOrigins } from 'vite';

export default defineConfig({
    server: {
        cors: {
            origin: [
                defaultAllowedOrigins,
                'my-app.com',
            ],
        }
    }
});

Future proofing question

Should we make the defaultAllowedOrigins constant an array? I can imagine that this might slowly get more things added to the regex and it could become to maintain as a single regex expression. Having it as an array from the outset might help future proof if you ever want to split the pattern into diffe.

origin: [
    ...defaultAllowedOrigins,
    'my-app.com',
],

[patak-dev]

We discussed today that it's better to merge this one in 6.1, but let's us know if you need it before and we can recheck this.

[timacdonald]

No rush on my end. I've copied the value over manually for now.

This will just help with future proofing.