proposal: add option to load custom icons from external sources

We are trying (@antfu and me) to make a universal toolkit to share some logic to allow load svg icons from any source on unocss and unplugin-icons.

Instead creating a shared repo to use it on both projects, I think the better place will be here: this way you can also use it for testing before adding new collections or new icons to existing collections.

The logic to be included here, right now, is only supported on unplugin-icons:

• load from external sources (network or filesystem): currently only built-in support for filesystem with optional transform callback for all icons
• load from inline sources: all collections and their icons on the configuration

The main problem here is that all collection/icon loading logic is not asynchronous:

• https://github.com/iconify/iconify/blob/master/packages/utils/src/icon-set/get-icon.ts
• https://github.com/iconify/iconify/blob/master/packages/utils/src/icon-set/get-icons.ts

On unplugin-icons we load the collections/icons using async tasks:

• https://github.com/antfu/unplugin-icons/blob/main/src/core/modern.ts#L57
• https://github.com/antfu/unplugin-icons/blob/main/src/core/modern.ts#L25

/cc @cyberalien

[cyberalien]

Sure, that can be done.

However, I have few concerns:

1. If types are to be added to utils, there should be code that works with those types, so functions that use it should be added as well.
2. Utils should not have any dependencies, except for types. It is meant to be a package that can be used in any environment. Helper functions such as camelize and pascalize can be added to utils. Awaitable type can be imported. Node specific functions, such as ones that work with file system, should be isolated in separate files to make sure they are not imported by accident.

[cyberalien]

I’ve just made new branch dev/more-utils from master