Describe your motivation

The V24 theme folder's components subfolder is used to inject stylesheets into the shadow DOM of custom elements whose element names match that of the stylesheet.

In V25, the theme folder in its entirety is deprecated (but still supported), and shadow DOM styling is considered a legacy feature.

While the theme folder, and the components-folder-based injection mechanism will still be supported for backward compatibility, it should be disabled by default.

Describe the solution you'd like

• A feature flag for enabling the components folder's shadow DOM injection.
• Perferrably, a warning shown if the theme/sometheme/components/vaadin-something.css structure is found in the project and the flag is not enabled.