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.

shared/lib/autocolors.ts

@@ -43,8 +43,10 @@ function areAutoColorsValid (autocolors: AutoColors): true | string[] {
   return true
 }
 
+export type {
+  AutoColors
+}
 export {
-  AutoColors,
   isAutoColorsAvailable,
   areAutoColorsValid
 }