Add console interface: calculator loop, ASCII title and tests

MIL-003 Console interface.

- Add calculator() with recursive restart: y continues with the result,
  n starts a new calculation, q quits
- Show one ASCII title at start: a calculator with the lettering to
  its right
- Re-prompt on invalid numbers, operations and answers; restart on
  division by zero
- Add __main__.py for python -m calc and the calc console script
- Add interface tests with input() fed through monkeypatch

Closes #22
Closes #23
Closes #24
Closes #25

Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4

pyproject.toml

@@ -14,6 +14,9 @@ dependencies = []
 [project.optional-dependencies]
 dev = ["pytest>=8"]
 
+[project.scripts]
+calc = "calc.calculator:main"
+
 [tool.setuptools.packages.find]
 where = ["src"]