[JS BE] Make sourceMap generation more precise

- Don't produce mapping for closing bracket in case of expressionBody
 - Map Kt*Function declaration into corresponding js fun declaration
 - Update test data & add new test

js/js.translator/testData/sourcemap/expressionBody.kt

@@ -0,0 +1,13 @@
+package foo
+
+fun test1() = "O"
+fun test2(s: String): String {
+    return s
+}
+
+fun test(s:String) =
+    test1() +
+            test2(s)
+
+fun box()
+        = test("K")