Add sharezone_lints to sharezone_about_page_addon package

[nilsreichardt]

This PR adds sharezone_lints to the sharezone_about_page_addon package and fixes the following lint warnings:

Analyzing sharezone_about_page_addon...                                 

   info • Unnecessary 'late' modifier • lib/game/custom/util.dart:11:13 • unnecessary_late
   info • Constructors for public widgets should have a named 'key' parameter • lib/sharezone_about_page_addon.dart:70:7
          • use_key_in_widget_constructors
   info • Invalid use of a private type in a public API • lib/sharezone_about_page_addon.dart:72:3 •
          library_private_types_in_public_api

3 issues found. (ran in 1.0s)

Part of #37

[github-actions]

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