icon user exp improve

[vthinkxie]

• use forChild to register icons to ng-zorro-antd submodules
• change NZ_CONFIG to the params of the forRoot(icons, config)

[wzhudev]

I don't think it's a good idea to make properties of NZ_CONFIG.icon to be a parameter of NzIconModule.forRoot. Anyway, if users want to change the config at runtime, they still need to call NzConfigService or NzIconService. I am afraid we are introducing unnecessary complexity.

[vthinkxie]

forRoot is just a syntactic sugar IMO, since we have already provided the forChild function, it makes sense for the users to use the forRoot
https://angular.io/guide/singleton-services#the-forroot-pattern
if the user need to change the config, they still can use the NzConfigService or NzIconService.
but I believe most users won't need to change it at the runtime.
ref
https://github.com/vthinkxie/di-factory-lazy-loading/blob/master/src/app/icon/icon.module.ts#L26

[wzhudev]

Already implemented in #4711. I am closing this.