Map and look up unit abbreviations by strings instead of enums

[angularsen]

Blocks #1181

QuantityType enum is already deprecated and will be removed in #982 in order to better support third-party quantities and units.

The same could be done for the 100+ unit enums, like LengthUnit

Instead of

string abbreviation = UnitAbbreviationsCache.Default.GetDefaultAbbreviation(unitInfo.GetType(), (int)unitInfo);

We could support, without breaking change:

string abbreviation = UnitAbbreviationsCache.Default.GetDefaultAbbreviation("Length", "Millimeter");

Optionally, we could deprecate the enum-based overloads and remove them in #982 or future major version bump.

Related to discussion: https://github.com/angularsen/UnitsNet/issues/1062#issuecomment-1073219229

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