Return service provider instance from App::register()

Which allow 4.0 user to manually run boot()

Signed-off-by: crynobone <crynobone@gmail.com>
fix phpdoc

Signed-off-by: crynobone <crynobone@gmail.com>
Merge pull request #2799 from crynobone/patch/return-service-provider

Return service provider instance from App::register()