[8.0] Fix merging config

[ElfSundae]

mergeConfigFrom should be called in the register method, it means once the package being registered, its config are ready to use. Then other services can access them safely without caring about the order of application services bootstrap.

This is also mentioned in the official documentation:

To merge the configurations, use the mergeConfigFrom method within your service provider's register method.

publishes is useful only in the console application, and in the console application all services will be booted, so it does not matter putting publishes whether in register or boot method. Some core services also put it in register to make code organized, such as MailServiceProvider::registerMarkdownRenderer.