Rule to avoid keyword self

[GuySartorelli]

We've recently updated our coding standards to avoid using self.

We should add a linting rule that enforces this and fails if self is used.
There is a documented exception to this with traits.

Acceptance criteria

• Outside of traits, any use of the keyword self results in CI failure
• In traits, self is only directly used to get the class name, const values, or as type hints. For everything else, self::class is used first, e.g. self::class::myMethod()

PR

• NEW Add rule to avoid use of "new" keyword #9