Generate units as UnitOf<TQuantity>, such as LengthUnits.Meter

[tmilnthorp]

Step 1 of the proposal in #1740: generate every unit as a UnitOf<TQuantity>, such as LengthUnits.Meter, next to the unit enums. Opened as a draft until the direction in #1740 is agreed.

Length length = Length.FromMeters(3);
length.As(LengthUnits.Foot);            // same as length.Feet
Length.Info.From(2, LengthUnits.Foot);  // 2 ft

var furlong = new UnitOf<Length>("Furlong", "Furlongs", 201.168);
length.As(furlong);

• One static class per quantity (LengthUnits), with a static readonly UnitOf<Length> field per unit holding its names, base units and exact conversions. UnitOf itself knows nothing about enums or UnitKey.
• Default mappings built from these: new (LengthUnit.Foot, LengthUnits.Foot), so each unit's data is generated in one place.
• As/From look built-in units up in the quantity's QuantityInfo, so they convert exactly like their enum values, configured conversions included, and From keeps the quantity in that unit. Custom units still use their own conversions.
• UnitOf<TQuantity> derives from a new non-generic UnitOf. It holds the data, so the code isn't compiled again for each quantity. It's also the type a future IQuantity.Unit would return.

Purely additive. It builds on #1737 and #1738, which should be merged first.

net10.0, Release	master	this PR
UnitsNet.dll	1,885,696 B	1,750,528 B
First ToString(), cold process	381 ms	360 ms
All quantity infos, cold process	355 ms, 1,564 KB allocated	345 ms, 1,894 KB allocated

Against #1737 alone, the generated units cost about 59 KB, plus about 13 ms and 436 KB of allocations when every quantity is created.

Part of #1740.

🤖 Generated with Claude Code

https://claude.ai/code/session_012XKhDsyHDc5BHrmibxScqG

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98%. Comparing base (3bf1ef4) to head (6eff3d9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #1741    +/-   ##
=======================================
- Coverage      98%     98%    -1%     
=======================================
  Files         515     645   +130     
  Lines       24092   24137    +45     
=======================================
+ Hits        23692   23736    +44     
- Misses        400     401     +1     

Flag	Coverage Δ
net48	97% <100%> (+<1%)	⬆️

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