Move linters into seperate module.

Give linters an abstract type to inherit, since they share an interface.
Add empty README

printer-linter/src/printerlinter/linters/diagnostic_generator.py

@@ -0,0 +1,14 @@
+from abc import ABC, abstractmethod
+from typing import Iterator
+
+from ..diagnostic import Diagnostic
+
+
+class DiagnosticGenerator(ABC):
+    def __init__(self, file, settings) -> None:
+        self._settings = settings
+        self._file = file
+
+    @abstractmethod
+    def check(self) -> Iterator[Diagnostic]:
+        pass