JS: apply boxing when initializing val parameter of type Char

See KT-20854

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

@@ -0,0 +1,16 @@
+// EXPECTED_REACHABLE_NODES: 1127
+class A(val x: Char)
+
+fun typeOf(x: dynamic): String = js("typeof x")
+
+fun box(): String {
+    val a = A('0')
+
+    var r = typeOf(a.asDynamic().x)
+    if (r != "object") return "fail1: $r"
+
+    r = typeOf(a.x)
+    if (r != "number") return "fail2: $r"
+
+    return "OK"
+}