fix: update UnitConversionRate type and create UnknownUnitConversionRate type

Author: RebeccaStevens

src/core.ts

@@ -58,7 +58,9 @@ export type UnitFrom<
  *
  * @group Unit Generators
  */
-export type UnitConversionRate<M extends UnitSubvalues> = Unit<{}, M>;
+export type UnitConversionRate<M extends UnitSubvalues> = number & {
+  readonly __uom_types__unit_meta: UnitMeta<M>;
+};
 
 /**
  * Create a {@link UnitConversionRate} from already existing {@link UnitMeta}.
@@ -107,6 +109,15 @@ export type UnknownAbstractUnit = number & {
   readonly __uom_types__unit_class: UnknownUnitClass;
 };
 
+/**
+ * An {@link UnitConversionRate} that we don't know anything about.
+ *
+ * @group Unknown Units
+ */
+export type UnknownUnitConversionRate = number & {
+  readonly __uom_types__unit_meta: UnknownUnitMeta;
+};
+
 /**
  * A {@link UnitClass} that we don't know anything about.
  *

src/index.ts

@@ -15,6 +15,7 @@ export type {
   UnknownAbstractUnit,
   UnknownUnit,
   UnknownUnitClass,
+  UnknownUnitConversionRate,
   UnknownUnitMeta,
 } from "./core";
 export type {