feat(render): add sheet grid text rendering

lib/src/grid.dart

@@ -0,0 +1,72 @@
+import 'address.dart';
+import 'sheet.dart';
+
+String renderGrid(Sheet sheet, {bool formulas = false, bool useFormat = true}) {
+  final dimensions = sheet.dimensions();
+  if (dimensions.rows == 0) return '(empty sheet)\n';
+
+  String contentAt(int col, int row) {
+    final address = CellAddress(col, row);
+    if (formulas) return sheet.rawAt(address);
+    return useFormat ? sheet.displayAt(address) : sheet.valueAt(address).display();
+  }
+
+  final colWidths = <int>[];
+  for (var col = 0; col < dimensions.cols; col++) {
+    var width = columnLabel(col).length;
+    for (var row = 0; row < dimensions.rows; row++) {
+      final length = contentAt(col, row).length;
+      if (length > width) width = length;
+    }
+    colWidths.add(width);
+  }
+
+  final gutter = dimensions.rows.toString().length;
+  final buffer = StringBuffer();
+
+  buffer.write(' ' * gutter);
+  for (var col = 0; col < dimensions.cols; col++) {
+    buffer.write(' | ');
+    buffer.write(columnLabel(col).padLeft(colWidths[col]));
+  }
+  buffer.writeln();
+
+  buffer.write('-' * gutter);
+  for (var col = 0; col < dimensions.cols; col++) {
+    buffer.write('-+-');
+    buffer.write('-' * colWidths[col]);
+  }
+  buffer.writeln();
+
+  for (var row = 0; row < dimensions.rows; row++) {
+    buffer.write((row + 1).toString().padLeft(gutter));
+    for (var col = 0; col < dimensions.cols; col++) {
+      buffer.write(' | ');
+      buffer.write(contentAt(col, row).padLeft(colWidths[col]));
+    }
+    buffer.writeln();
+  }
+
+  return buffer.toString();
+}
+
+String renderTextRows(List<List<String>> rows) {
+  if (rows.isEmpty) return '';
+  final columns = rows.map((row) => row.length).reduce((a, b) => a > b ? a : b);
+  final widths = List<int>.filled(columns, 0);
+  for (final row in rows) {
+    for (var c = 0; c < row.length; c++) {
+      if (row[c].length > widths[c]) widths[c] = row[c].length;
+    }
+  }
+  final buffer = StringBuffer();
+  for (final row in rows) {
+    final cells = <String>[];
+    for (var c = 0; c < columns; c++) {
+      final value = c < row.length ? row[c] : '';
+      cells.add(value.padRight(widths[c]));
+    }
+    buffer.writeln(cells.join('  ').trimRight());
+  }
+  return buffer.toString();
+}