Allow BuildKonfig to be marked with @JsExport

[dellisd]

This is a pretty niche use case, but with Kotlin 1.4's generated TypeScript definitions only top level declarations marked with @JsExport will be included.

In my case, I am building a web client in TypeScript (rather than use Kotlin/JS) that makes use of a shared multiplatform module that depends on some API keys that are set through a BuildKonfig object.

Naturally, this would depend on #31.

[yshrsmz]

Yes, this should be covered too.

According to the API document, expect declaration can not be exported for now.

So actual declaration in js sourceset should be annotated, and we need to find a way to check the target platform.

[yshrsmz]

So we need to

• annotate the common object with @JsExport if there is no target specific BuildKonfig configuration
• or annotate actual object in js sourceset