Add atomic unit of electric field gradient

[Sathya-S3]

Summary

This PR adds the atomic unit of electric field gradient to Pint's unit definitions.

Changes Made

• Added new dimension: [electric_field_gradient] = [energy] / [area] / [charge]
• Added atomic unit: atomic_unit_of_electric_field_gradient = E_h / a_0 ** 2 / e
• Added alias: a_u_efg for convenient usage
• Updated CHANGES file: Documented the addition for version 0.26

Checklist

• Closes Requesting addition of atomic_unit_electric_field_gradient to unit registry #2269
• Executed pre-commit run --all-files or pixi run lint --all-files with no errors
• The change is fully covered by automated unit tests
• Added an entry to the CHANGES file

[codspeed-hq]

Merging this PR will not alter performance

✅ 448 untouched benchmarks

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_{Comparing CCP-NC:master (7836086) with master (6049a9c)}