feat(model): add workbook json serialization

lib/src/workbook_json.dart

@@ -0,0 +1,77 @@
+import 'dart:convert';
+
+import 'address.dart';
+import 'format.dart';
+import 'names.dart';
+import 'workbook.dart';
+
+const int workbookJsonVersion = 1;
+
+String encodeWorkbookJson(Workbook workbook) {
+  final sheets = <Map<String, dynamic>>[];
+  for (final sheet in workbook.sheets) {
+    final cells = <String, String>{};
+    final formats = <String, String>{};
+    for (final entry in sheet.cells.entries) {
+      final cell = entry.value;
+      if (cell.isOccupied) cells[entry.key.label] = cell.raw;
+      final format = cell.format;
+      if (format != null) formats[entry.key.label] = format.toCode();
+    }
+    sheets.add({
+      'name': sheet.name,
+      'cells': cells,
+      if (formats.isNotEmpty) 'formats': formats,
+    });
+  }
+
+  final names = <String, String>{};
+  for (final entry in workbook.definedNames) {
+    names[entry.key] = _referenceText(entry.value);
+  }
+
+  final document = <String, dynamic>{
+    'version': workbookJsonVersion,
+    'sheets': sheets,
+    if (names.isNotEmpty) 'names': names,
+  };
+  return const JsonEncoder.withIndent('  ').convert(document);
+}
+
+Workbook decodeWorkbookJson(String source) {
+  final document = jsonDecode(source) as Map<String, dynamic>;
+  final workbook = Workbook();
+  final sheetsJson = (document['sheets'] as List).cast<Map<String, dynamic>>();
+  for (final sheetJson in sheetsJson) {
+    final sheet = workbook.addSheet(sheetJson['name'] as String);
+    final cells = (sheetJson['cells'] as Map).cast<String, dynamic>();
+    for (final entry in cells.entries) {
+      sheet.setCell(CellAddress.parse(entry.key), entry.value as String, recalculate: false);
+    }
+    final formats = sheetJson['formats'] as Map?;
+    if (formats != null) {
+      for (final entry in formats.cast<String, dynamic>().entries) {
+        sheet.setFormatAt(CellAddress.parse(entry.key), NumberFormat.fromCode(entry.value as String));
+      }
+    }
+  }
+
+  final names = document['names'] as Map?;
+  if (names != null) {
+    for (final entry in names.cast<String, dynamic>().entries) {
+      workbook.defineName(entry.key, entry.value as String);
+    }
+  }
+
+  workbook.recalculate();
+  return workbook;
+}
+
+String _referenceText(NameTarget target) {
+  switch (target) {
+    case CellName(:final sheet, :final address):
+      return "'${sheet.name}'!${address.label}";
+    case RangeName(:final sheet, :final start, :final end):
+      return "'${sheet.name}'!${start.label}:${end.label}";
+  }
+}