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

js/js.translator/testData/box/coercion/unitIsAs.kt

@@ -0,0 +1,14 @@
+// EXPECTED_REACHABLE_NODES: 1002
+var log = ""
+
+fun foo(): Unit {
+    log += "foo();"
+}
+
+fun box(): String {
+    if (foo() !is Any) return "fail1"
+    if (foo() as Any != Unit) return "fail2"
+    if (log != "foo();foo();") return "fail3: $log"
+
+    return "OK"
+}