refactor!: use `:global()` for custom styles

[metonym]

I'm currently working on rewriting the Carbon CSS optimize plugin: https://github.com/carbon-design-system/carbon-preprocess-svelte/pull/54

Here's one example of how much unused Carbon CSS can be eliminated (using #header-with-app-switcher).

Optimized assets/index-DekzpXBw.css
Before: 606.68 kB
After:   84.58 kB (-86.06%)

However, the current approach relies on explicit .bx-- selectors to avoid false positives.

This breaking change requires existing, custom styles to:

• Follow the .bx-- prefixing pattern
• Be globally scoped (otherwise, the compiler will generate a hashed selector name)