Add mechanism for type coercion in JS

Use it for char boxing/unboxing and unit materialization.
Possible to use for other purposes, for example, to add type checks
to dynamics.

See KT-18793, KT-17915, KT-19081, KT-18216, KT-12970, KT-17014,
KT-13932, KT-13930

compiler/testData/codegen/box/casts/lambdaToUnitCast.kt

@@ -1,6 +1,3 @@
-// IGNORE_BACKEND: JS
-// JS backend does not support Unit well. See KT-13932
-
 val foo: () -> Unit = {}
 
 fun box(): String {