Extract RuleDefinition as the base identity for rules

[vinistock]

Third step towards #1000

We need to share the concept of a rule across Rust and Ruby, the graph and the linter. However, Rule currently enforces that each descendant must implement lint. This is an issue for unification because graph and linter diagnostics are collected differently.

In reality, the common concept is not Rule, but a RuleDefinition. All we need is the identity of the rule: a name and the default severity. With this, we can treat all rules similarly for the configuration as it applies to the RuleDefinition and not the Rule.

In more abstract terms, this PR makes RuleDefinition the identity of a rule, while Rule is the execution abstraction for the definition.

[st0012]

IMO RuleDefinition is more like an interface then a base class, so using a module probably makes more sense?
And I think it should be kept under the Linter namespace.

[vinistock]

I moved it under the linter. However, after trying out the module approach, I find it a bit strange. I would keep it as a parent class that defines the identity of a rule.