Upgrade Doctrine CS, use only for new files

[Majkl578]

6.0.x-dev because I am going to use the new syntax for generics and intersections which only works in 6.0-dev.
Not applying to any code from 1.x era to avoid headaches during rebase conflicts. We will do it later for files that survive refactoring.

[Ocramius]

Lots and lots of excludes: is this worth doing in first place, if half the lib isn't covered at all?

[Majkl578]

It's for the new files we add, e.g. #247, new parser etc.

[alcaeus]

Agree with @Ocramius here: if we're only going to exclude most files, let's do this when we don't need to exclude everything. Just to understand the reasoning, why not wait until #247 is merged and then do this for the entire codebase?

[Majkl578]

@alcaeus Already described the situation in OP:

Not applying to any code from 1.x era to avoid headaches during rebase conflicts. We will do it later for files that survive refactoring.

[alcaeus]

After discussion in Slack, we've decided to hold back introducing doctrine/coding-standard until the initial refactoring is done: the current coding standard should not be applied to code that will not survive refactoring, but at the same time we also don't want to apply the coding standard to a small subset of files.

Instead, maintainers and containers are encouraged to make sure their code is formatted in compliance with doctrine/coding-standard. However, code reviews should refrain from enforcing doctrine/coding-standard until it is officially adopted in the repository and checked automatically.