Add Documentation comments to the generated translation file

[fa0311]

Motivation

I migrated from gen_l10n to slang, and the generated documentation is better in gen_l10n than in slang. Since I am a native Japanese speaker, I need to be able to quickly preview member variables in Japanese, not just their names.

gen_l10n generates the following.

  /// No description provided for @title.
  ///
  /// In en, this message translates to:
  /// **'Title'**
  ///
  /// In ja, this message translates to:
  /// **'タイトル'**
  String get title;

slang generates the following.

	String get title => 'title';