Add comment and return unit text in TerminalParsers.cs

Improved the handling of unit text in the `TerminalParsers.cs` file within the `Hl7.Cql.CqlToElm.Visitors` namespace. Added a comment noting that unit range validation is not yet implemented and updated the method to return a tuple containing the parsed decimal value and the unit text. This change enhances code clarity and ensures proper handling of unit text.

Author: baseTwo

Cql/Cql.CqlToElm/Visitors/TerminalParsers.cs

@@ -77,6 +77,8 @@ public static (decimal value, string unit) Parse(this cqlParser.QuantityContext
 
             // This is either a unit, or a datetimeprecision (which we parse as text here)
             var unitText = context.unit().STRING().ParseString() ?? context.unit().GetText();
+
+            // We should actually validate the range of units here, but for now we just return it as-is.
             return (decimalValue, unitText!);
         }