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/standardClasses/charArrayGetSet.kt

@@ -0,0 +1,26 @@
+// EXPECTED_REACHABLE_NODES: 997
+fun box(): String {
+    val a = CharArray(1)
+    val aType = jsTypeOf(a.asDynamic()[0])
+    if (aType != "number") return "fail1: $aType"
+
+    a[0] = 'Q'
+    val aType2 = jsTypeOf(a.asDynamic()[0])
+    if (aType2 != "number") return "fail2: $aType2"
+
+    val aType3 = jsTypeOf(a[0].asDynamic())
+    if (aType3 != "number") return "fail3: $aType3"
+
+    val b = Array<Char>(1) { 'Q' }
+    val bType = jsTypeOf(b.asDynamic()[0])
+    if (bType != "object") return "fail4: $bType"
+
+    b[0] = 'W'
+    val bType2 = jsTypeOf(b.asDynamic()[0])
+    if (bType2 != "object") return "fail5: $bType2"
+
+    val bType3 = jsTypeOf(b[0].asDynamic())
+    if (bType3 != "number") return "fail6: $bType3"
+
+    return "OK"
+}