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/settings.ts

@@ -205,3 +205,5 @@ function launchTests (): void {
 }
 
 launchTests()
+
+export {}