Use character based columns

Towards #74

Use `Characters` to operate based on grapheme clusters instead of code
units.

lib/src/file.dart

diff --git a/lib/src/file.dart b/lib/src/file.dart
index 473b8c1..047a644 100644
--- a/lib/src/file.dart
+++ b/lib/src/file.dart
@@ -5,6 +5,8 @@
 import 'dart:math' as math;
 import 'dart:typed_data';
 
+import 'package:characters/characters.dart';
+
 import 'location.dart';
 import 'location_mixin.dart';
 import 'span.dart';
@@ -184,7 +186,9 @@
       throw RangeError('Line $line comes after offset $offset.');
     }
 
-    return offset - lineStart;
+    final content = _decodedChars.sublist(lineStart, offset);
+    final characters = Characters(String.fromCharCodes(content));
+    return characters.length;
   }
 
   /// Gets the offset for a [line] and [column].
@@ -202,11 +206,16 @@
       throw RangeError('Column may not be negative, was $column.');
     }
 
-    final result = _lineStarts[line] + column;
-    if (result > length ||
-        (line + 1 < lines && result >= _lineStarts[line + 1])) {
+    final content = line < lines - 1
+        ? _decodedChars.sublist(_lineStarts[line], _lineStarts[line + 1])
+        : _decodedChars.sublist(_lineStarts[line]);
+    final characters = Characters(String.fromCharCodes(content));
+    if (characters.length < column) {
       throw RangeError("Line $line doesn't have $column columns.");
     }
+    final withinLineLength =
+        characters.take(column).toString().codeUnits.length;
+    final result = _lineStarts[line] + withinLineLength;
 
     return result;
   }

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 63a64b0..2b4e4f9 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -7,6 +7,7 @@
   sdk: ">=2.14.0 <3.0.0"
 
 dependencies:
+  characters: ^1.2.0
   collection: ^1.15.0
   path: ^1.8.0
   term_glyph: ^1.2.0