CsvFieldIndexer: Fix calculation of offsets with unicode characters.

Fixes #4

CsvLib.Tests/CsvFieldIndexerTests.cs

@@ -137,6 +137,39 @@ public class CsvFieldIndexerTests
         Assert.Equal(29, indexer.FieldIndex[1][3]);
     }
 
+    [Fact]
+    public void GenerateIndex__TwoLinesWithTwoQuotedColumnsWithUnicode__TwoRowsTwoFields()
+    {
+        // --- Arrange
+        StringReader sr = new("""
+                              "Hélló","Wórld"
+                              "Hélló","Wórld"
+                              """);
+
+        // --- Act
+        CsvFieldIndexer indexer = new();
+        indexer.GenerateIndex(sr);
+
+        // --- Assert
+
+        Assert.Equal(3, indexer.Index.Count);
+        Assert.Equal(0, indexer.Index[0]);
+        Assert.Equal(19, indexer.Index[1]);
+        Assert.Equal(38, indexer.Index[2]);
+
+        Assert.Equal(2, indexer.FieldIndex.Count);
+        Assert.Equal(4, indexer.FieldIndex[0].Count);
+        Assert.Equal(1, indexer.FieldIndex[0][0]);
+        Assert.Equal(7, indexer.FieldIndex[0][1]);
+        Assert.Equal(11, indexer.FieldIndex[0][2]);
+        Assert.Equal(16, indexer.FieldIndex[0][3]);
+        Assert.Equal(4, indexer.FieldIndex[1].Count);
+        Assert.Equal(20, indexer.FieldIndex[1][0]);
+        Assert.Equal(26, indexer.FieldIndex[1][1]);
+        Assert.Equal(30, indexer.FieldIndex[1][2]);
+        Assert.Equal(35, indexer.FieldIndex[1][3]);
+    }
+
     #endregion GenerateIndex
 
 }