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/tryWithEmptyCatch.kt

@@ -0,0 +1,19 @@
+// EXPECTED_REACHABLE_NODES: 1004
+fun test(x: Int): Any {
+    return try {
+        if (x % 2 == 0) throw RuntimeException()
+        x
+    }
+    catch (e: RuntimeException) {
+    }
+}
+
+fun box(): String {
+    val a = test(1)
+    if (a != 1) return "fail1: $a"
+
+    val b = test(2)
+    if (b != Unit) return "fail2: $b"
+
+    return "OK"
+}