JS backend: fix translation `when` statement -- now WhenTranslator returns JsBlock instead of hackish adding statements to context block.

It fixed the compiler crashing when translating `for`/`when`/`if` with `when` statement(KT-5234) and KT-5058.

 #KT-5234 fixed
 #EA-57346 fixed
 #KT-5058 fixed