feat(/convert): add more unit types

[kyranet]

Add the following types:

• Angle
• Voltage
• Resistance
• Digital storage

With #201 (complex type support), we can support the following types:

• Speed (Distance / Time)
• Frequency (1 / Time)
• Force (N = km⋅m⋅s⁻² or F = ma)
• Pressure (Pa = kg⋅m⁻¹⋅s⁻² or p = F / A)
• Energy (J = kg⋅m²⋅s⁻²)
• Power (W = kg⋅m²⋅s⁻³, or P = E/t, F·v, V·I, or τ·ω.
  • And all sub-units such as Torque and Angular velocity.