chore: add example to demonstrate some features

examples/02_demonstration/demo.midas

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+predicate in_range(min: float, max: float)(v: float) = min <= v & v <= max
+predicate is_ratio = in_range(0, 1)
+
+type Money = float
+type Price[T <: Money] = T where _ >= 0
+
+type EUR = Money
+type USD = Money
+
+type Reduction = float where is_ratio(_)

examples/02_demonstration/demo.py

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+from typing import TypeVar, cast
+
+from demo_stubs import EUR, USD, Money, Price, Reduction
+
+T = TypeVar("T", bound=Money)
+
+
+def apply_reduction(amount: Price[T], reduction: Reduction) -> Price[T]:
+    return cast(Price[T], (1.0 - reduction) * amount)
+
+
+a1 = cast(Price[EUR], 3.2)
+a2 = cast(Price[USD], 10.4)
+r1: Reduction = cast(Reduction, 0.2)
+
+print(apply_reduction(a1, r1))
+print(apply_reduction(a2, r1))
+
+# a3 = a1 + a2