feat(preset): add @mdi/js support

[userquin]

Description

This PR includes:

• change the context to await when preset definition is an async function, we're using dynamic import to import all the icons from @mdi/js module
• add usage in Svelte example

Linked Issues

closes #446

Additional context

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[antfu]

I am not sure to support this specific package, while we have more generic solutions like https://github.com/unplugin/unplugin-icons. We can't have presets for every single package no npm.

Btw, it's probably under-documented, but we do support auto crawling exports for packages, you can try (haven't tested):

presets: [
  { package: '@mdi/js' }
]

https://github.com/unjs/unimport/blob/b1f1d33d6a26e16857315818e84b6f345e12ef0d/src/preset.ts

[userquin]

I Will check it, if working I Will add the entry to the docs.