Magnetic Variation from WMM (World Magnetic Model) component

**Describe the problem you have/What new integration you would like**

Magnetic Variation from WMM (World Magnetic Model) component which calculates it based on location

This code can be re-used:

https://github.com/DavidArmstrong/WMM_Tinier

**Please describe your use case for this integration and alternatives you've tried:**

**Additional context**

No project files, tests, or entry points are identified. Start by reviewing the linked WMM_Tinier implementation and the ESPHome component conventions; the work is done when a defined component calculates magnetic variation from location and its integration scope is agreed.

Written by the indexing model from the issue text.