feat(nuxt): namespace global variables for multi-app support

[nicolaspayot]

🔗 Linked issue

2nd task of #21635. Also linked to #25336 although it doesn't completely remove window.__NUXT__.

📚 Description

This PR updates window.__NUXT__ global variable to handle multi-app payloads and config by storing the data under their appId.

Before:

window.__NUXT__ = { config: {...}, data: {...}, ...}

After:

window.__NUXT__ = {
  'nuxt-app-1': { config: {...}, data: {...}, ...} }
}

It also replaces scripts global IDs __NUXT_DATA__ and __NUXT_LOGS__ with data attributes:

Before:

<script type="application/json" id="__NUXT_DATA__">...</script>
<script type="application/json" id="__NUXT_LOGS__">...</script>

After:

<script type="application/json" data-nuxt-data="nuxt-app-1">...</script>
<script type="application/json" data-nuxt-logs="nuxt-app-1">...</script>

This will eventually allows the use of multiple Nuxt applications on a same page.

📝 To do:

• Protect these changes under a future / experimental flag
• Handle window.__NUXT__ update in nuxt/test-utils. See feat: add multi-app support for global var __NUXT__ test-utils#858

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[TheAlexLichter]

I'm still wondering if we have to set the payload here / why it was marked for compat only?

[typed-sigterm]

I have an idea - just an idea - perhaps we don't need to differentiate between the single app and multiple apps scenarios, as a single app is just a special case of multiple apps. Given the major version 4.0, this breaking change might be acceptable (it has been removed for the user anyway)

[danielroe]

Agreed, I would be happy to merge that just into v4.