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/buttons.ts

@@ -103,6 +103,8 @@ export {
   closeSVG,
   openChatSVG,
   openBlankChatSVG,
-  shareChatUrlSVG,
+  shareChatUrlSVG
+}
+export type {
   SVGButton
 }