Prevent leaking top-level

[stuebingerb]

aPureBase/KGraphQL#17

Prevent that top-level declarations (like fun Collection<*>.toJsonElement()) leak unintentionally outside the library.

Maybe use https://github.com/Kotlin/KEEP/blob/master/proposals/explicit-api-mode.md (maybe start with explicitApiWarning()) in combination with #210

 kotlin {
+    explicitApi()
     jvmToolchain(17)
     compilerOptions {
         apiVersion.set(KotlinVersion.KOTLIN_2_1)
         languageVersion.set(KotlinVersion.KOTLIN_2_1)
     }
 }