build: Add unifiedpush_ui package

[baltevl]

Add unifiedpush_ui as a part of unifiedpush was split off into it.

Thank you so much for your contribution to FluffyChat ❤️❤️❤️

Please make sure that your Pull Request meet the following acceptance criteria:

• Code formatting and import sorting has been done with dart format lib/ test/ and dart run import_sorter:main --no-comments
• The commit message uses the format of Conventional Commits
• The commit message describes what has been changed, why it has been changed and how it has been changed
• (Not applicable) Every new feature or change of the design/GUI is linked to an approved design proposal in an issue
• (Not applicable) Every new feature in the app or the build system has a strategy how this will be tested and maintained from now on for every release, e.g. a volunteer who takes over maintainership

Pull Request has been tested on:

• Android (x86, Emulator)
• iOS
• Browser (Chromium based)
• Browser (Firefox based)
• Browser (WebKit based)
• Desktop Linux
• Desktop Windows
• Desktop macOS

[krille-chan]

LGTM thank you very much :)

[krille-chan]

@baltevl could you sign your commit? Signed commits is configured as a requirement in this repository

[baltevl]

Done :)