Implement the UI components required by the BlockNote editor using XWiki's design system

For the moment I implemented only the button UI component, to prove the that system works. See https://github.com/xwiki/xwiki-platform/commit/37706f9bac922ec64ba902c02772a0f95e72be76 . I'll open separate issues for the remaining UI components that are needed by BlockNote.

Note that the initial plan was to use Cristal's abstract design system in BlockNotes's UI (menus, toolbars, popups, dialogs, etc.), but we encountered issues with loading Vue components (e.g. buttons from Cristal) inside React (BlockNote's UI) inside another Vue component (the one loading the editor). In the end most of BlockNote's UI won't be using Cristal's abstract design system, so the need to implement Cristal's UI components in XWiki is less important.