[Console] Define isVerbose(), etc. methods in OutputInterface

Could / should the ```Symfony\Component\Console\Output\OutputInterface``` define the methods ```isQuiet()```, ```isVerbose()```, ```isVeryVerbose()``` and ```isDebug()```?

Because the Interface already defines the verbosity constants and the ```getVerbosity()``` method, it will probalby be the right place to do so...

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 
We don't add that yet.