Move the ES2015 compilation target setting to the demo project because it's only needed in the app project

The `app.js` bundle size is still the reduced one 1.1 MiB after this change.

Author: ShreckYe

buildSrc/src/main/kotlin/common-conventions.gradle.kts

@@ -38,10 +38,6 @@ kotlin {
     js {
         // The project works without this, but it can be added to avoid potential issues.
         browser()
-
-        compilerOptions {
-            target.set("es2015")
-        }
     }
 
 

demo/build.gradle.kts

@@ -40,6 +40,10 @@ kotlin {
             }
         }
         binaries.executable()
+
+        compilerOptions {
+            target.set("es2015")
+        }
     }
 
     sourceSets {