JS: fix char to string convertion in string templates (#KT-17966 fixed)

js/js.translator/testData/box/char/charInStringTemplate.kt

@@ -0,0 +1,24 @@
+package foo
+
+var log = ""
+
+fun sideEffects(c: Char?, msg: String): Char? {
+    log += msg
+    return c
+}
+
+fun box(): String {
+    val name = run { "John" }        // Force compiler to actually do some concatenation
+    assertEquals("${'$'}name = $name", "\$name = John")
+
+    val ch1: Char? = null
+    assertEquals("${ch1}name = $name", "nullname = John")
+
+    val ch2: Char? = '$'
+    assertEquals("${ch2}name = $name", "\$name = John")
+
+    assertEquals("${sideEffects('$', "1")!!}${sideEffects('n', "2")}ame = $name", "\$name = John")
+    assertEquals(log, "12")
+
+    return "OK"
+}