Add CubicAngstrom to Volume

[simone-russo-AP]

Summary
In crystallography, chemistry, and materials science, volumes are often expressed in cubic ångströms (ų). Currently, UnitsNet does not include this unit in Volume. It would be very useful to support it natively.

Details

• Definition:
  1 Å = 1×10⁻¹⁰ m
  → 1 ų = (1×10⁻¹⁰ m)³ = 1×10⁻³⁰ m³

• Proposed addition to Volume.json:

{
  "SingularName": "CubicAngstrom",
  "PluralName": "CubicAngstroms",
  "BaseUnits": {
     "L": "Angstrom"
   },
  "FromUnitToBaseFunc": "{x} * 1E-30",
  "FromBaseToUnitFunc": "{x} / 1E-30",
  "Localization": [
    {
      "Culture": "en-US",
      "Abbreviations": [ "ų" ]
    }
  ]
}

Motivation:

• ų is a standard unit in CIF (Crystallographic Information Files).
• Many databases (ICSD, COD, ICDD) use ų for unit cell volume.
• Having it directly available improves readability and avoids manual conversions.

Example usage

double vAngstrom3 = 500;
Volume v = Volume.FromCubicAngstroms(vAngstrom3);

Console.WriteLine(v.CubicNanometers); // 0.5
Console.WriteLine(v.CubicMeters);     // 5e-28

[angularsen]

Sounds good to me, willing to attempt a PR?
https://github.com/angularsen/UnitsNet/wiki/Adding-a-New-Unit