feat: unit system refactor

[nstarman]

This PR refactors the unit systems to be more class-based, allowing for static analysis of the unit systems. For users the focus shifts from the UnitSystem class to the unitsystem function which can dynamically create new unit systems classes, or use statically-defined ones.

Features:

• dataclass constructions
• static analysis

[codecov]

Codecov Report

Attention: Patch coverage is 98.73418% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.93%. Comparing base (b81a59f) to head (32d000a).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/unxt/_unxt/unitsystems/base.py	93.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
+ Coverage   89.91%   90.93%   +1.02%     
==========================================
  Files          28       31       +3     
  Lines        1487     1545      +58     
==========================================
+ Hits         1337     1405      +68     
+ Misses        150      140      -10     

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