Add tests for all justfile features

.coveragerc

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+# https://coverage.readthedocs.io/en/stable/config.html
+
+[run]
+relative_files = True
+
+[report]
+exclude_lines =
+    # Have to re-enable the standard pragma
+    pragma: no cover
+
+    # Don't complain if tests don't hit defensive assertion code
+    raise AssertionError
+    raise NotImplementedError
+    raise RuntimeError
+    raise ValueError
+
+    # Don't complain if non-runnable code isn't run
+    if __name__ == .__main__.:
+    # main\(
+
+    # Ignore unused conditionals
+    pass

mypy.ini

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+[mypy]
+
+[mypy-parsy]
+ignore_missing_imports = True