WasmImport annotated functions:
* Restricted to top-level external functions
* Only supports primitive numbers, booleans and Unit
(as return type only) in its signature
* Can't have default parameter values
* Can't have vararg parameters
@JsExport is only allowed on top-level functions in K/Wasm
Restrictions for nested function are handled by reusing K/JS diagnostics
Restrictions for classes and properties are handled by specifying
annotation target in stdlib
Prohibit:
- external enum class
- external tailrec fun
- external suspend fun
- external lateinit var
Add tests for other external diagnostics inherited from K/JS
Svyatoslav Kuzmich