translation: Add i18n for the Chinese language

i18n/zh-cn/cosmic_files.ftl

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+empty-folder = 空文件夹
+empty-folder-hidden = 空文件夹（包含隐藏文件）
+trash = 垃圾
+
+# Context Pages
+
+## Properties
+properties = 属性
+
+## Settings
+settings = 配置
+
+### Appearance
+appearance = 外观
+theme = 主题
+match-desktop = 搭配桌面
+dark = 深色
+light = 浅色
+
+# Context menu
+new-file = 新文件
+new-folder = 新建文件夹
+copy = 复制
+paste = 粘贴
+select-all = 全选
+move-to-trash = 删除