feat(file_utils): add support for CSV files

slang/lib/builder/utils/file_utils.dart

@@ -44,7 +44,55 @@ class FileUtils {
         }
         return json2yaml(content, yamlStyle: YamlStyle.generic);
       case FileType.csv:
-        throw UnimplementedError('CSV is not supported yet');
+        String escapeRow(String value) {
+          final escaped = value.replaceAll('"', '""');
+          if (escaped.contains(RegExp(r'[,"]'))) {
+            return '"$escaped"';
+          }
+          return escaped;
+        }
+
+        Map<String, String> encodeRow({
+          String key = '',
+          required dynamic value,
+        }) {
+          if (value is Map) {
+            final keyPrefix = key.isEmpty ? '' : '$key.';
+            return value.map((k, v) {
+              final map = encodeRow(key: '$keyPrefix$k', value: v);
+              final mapEntry = map.entries.first;
+
+              return MapEntry(mapEntry.key, mapEntry.value);
+            });
+          } else if (value is String) {
+            return {key: escapeRow(value)};
+          }
+          return {};
+        }
+
+        final Map<String, Map<String, String>> columns = {};
+        if (content.containsKey(INFO_KEY)) {
+          final info = content.remove(INFO_KEY);
+          columns[INFO_KEY] = {INFO_KEY: escapeRow(info.join('\\n'))};
+        }
+        content.entries.forEach((e) {
+          columns[e.key] = encodeRow(value: e.value);
+        });
+
+        // get all translation keys
+        final translationKeys = columns.values
+            .map((e) => e.entries.map((e) => e.key))
+            .expand((e) => e)
+            .toSet();
+
+        final headers = ['key', ...columns.keys].join(',');
+        final rows = translationKeys.map((key) {
+          final values =
+              columns.values.map((e) => e.containsKey(key) ? e[key] : '');
+          return "$key,${values.join(',')}";
+        });
+
+        return "$headers\n${rows.join('\n')}";
       case FileType.arb:
         // this encoder does not append \n automatically
         return JsonEncoder.withIndent('  ').convert(content) + '\n';