migrate js-stubs from kotlin-js to mpp with target js

[robstoll]

Platform (all, jvm, js): all
Extension (none, kotlin 1.3): none

Code related feature

When executing ./gradlew :js-stubs:build you can see the following warning in the output:

> Configure project :js-stubs
w: 'kotlin-js' Gradle plugin is deprecated and will be removed in the future. 
Please use 'kotlin("multiplatform")' plugin with a 'js()' target instead. See the migration guide: https://kotl.in/t6m3vu

Migrate misc/js-stubs/build.gradle.kts accordingly

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[chaitanyason9]

I'll work on this

[robstoll]

@chaitanyason9 do you need help?