feat: native unit support

**BEFORE**:
```python
# Native unit support
velocity = 5 m/s
force = 10 N = mass * acceleration
energy = 3 kJ -> eV # Unit conversion

def calculate_momentum(mass: kg, velocity: m/s) -> kg*m/s:
return mass * velocity
```

**AFTER**:
```python
# After (transpiled.py)
from vyper.units import Unit, Quantity
from typing import Any

def calculate_kinetic_energy(
mass: Quantity,
velocity: Quantity
) -> Quantity:
mass_val = mass.to('kg').magnitude
velocity_val = velocity.to('m/s').magnitude
return Quantity(0.5 * mass_val * velocity_val**2, 'joule')

result = calculate_kinetic_energy(
mass=Quantity(20, 'kg'),
velocity=Quantity(10, 'm/s')
).to('kJ')
```