Add WelcomeBanner.width extension API (#3663)

* Allow to add WelcomeBannerRegistration.width and use that to caculate to width of Banner container

Signed-off-by: Hung-Han (Henry) Chen <chenhungh@gmail.com>

* Export at const

Signed-off-by: Hung-Han (Henry) Chen <chenhungh@gmail.com>

src/extensions/registries/welcome-banner-registry.ts

@@ -30,6 +30,10 @@ export interface WelcomeBannerRegistration {
    * The banner component to be shown on the welcome screen.
    */
   Banner?: React.ComponentType
+  /**
+   * The banner width in px.
+   */
+  width?: number
 }
 
 export class WelcomeBannerRegistry extends BaseRegistry<WelcomeBannerRegistration> { }