Use ESBuild for frontend build.

Replacing webpack by ESBuild for client code compilation (as in the
official quickstart plugin).

So we can remove deprecated webpack dependencies.

Note: webpack is still used for ConverseJS build.
This may be removed soon.

Related to issue #122.

client/videowatch/button.ts

@@ -50,7 +50,9 @@ function displayButton ({
   buttonContainer.append(button)
 }
 
+export type {
+  displayButtonOptions
+}
 export {
-  displayButtonOptions,
   displayButton
 }