Refactor/core

[Khabib73]

implementation of the new architecture. Adding base classes to core.

Abstraction	Class	Responsibility
Rule	Rule / NodeRule / CustomRule	Describes the rule: code, name, node type, predicate function
Checker	BaseChecker	A group of related rules (e.g all docstrings))
Registry	RuleRegistry	Stores all rules, allows --select/--ignore
Orchestrator	Linter	Goes through files, launches checkers, collects violations
Reporter	Reporter	Accumulates violations, sorts, formats output