[1.0.0-beta1] UI Styles & UI Skins must be soft depedencies

Problem/Motivation

Those 2 modules are "hard" dependencies in info.yml because of:

• UiStylesPanel island plugin and the related BlockStylesForm (which may be removed by 📌 [1.0.0-beta1] Remove Island's specific Form classes Active )
• UiSkinsPanel island plugins and the related CssVariablesForm (which may be removed by 📌 [1.0.0-beta1] Remove Island's specific Form classes Active )
• the use of Drupal\ui_styles\Render\Element in BuilderPanel island plugin

However, not every projects needs styles and css variables.

Proposed resolution

Can we make them "soft" dependencies?

We may need to leverage mechanisms like \Drupal::service('module_handler')->moduleExists('devel')) in ::isApplicable() method

The BuilderPanel case may be more complex.