Restructure solver modules with classes and unittest suites

puzzles/_solver.py

@@ -0,0 +1,12 @@
+from abc import ABC, abstractmethod
+
+
+class Solver(ABC):
+    @abstractmethod
+    def __init__(self, puzzle_input: str): ...
+
+    @abstractmethod
+    def solve_p1(self) -> int: ...
+
+    @abstractmethod
+    def solve_p2(self) -> int: ...