fix: #3632149 Conflict with ui_skins < 1.2

Michael Zemuirequested to merge
issue/display_builder-3632149:3632149-servicenotfoundexception-cssvariablepluginmanagerinterface-with into 1.0.x

DesignTokensPanel gets CssVariablePluginManagerInterface whenever ui_skins is installed; that service only exists in ui_skins 1.2+. ui_skins ^1.2 was only declared in require-dev, so Composer kept older versions and every builder page failed with ServiceNotFoundException.

Closes #3632149

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