Generate relative paths in JS source maps

Also, add CLI options to manipulate prefixes of path

See KT-4078

compiler/testData/cli/js/sourceMapRoot/foo/baz/file4.kt

@@ -0,0 +1,3 @@
+fun j(): Float {
+    return 3.14159f
+}

compiler/testData/cli/js/sourceMapRoot/foo/file1.kt

@@ -0,0 +1,3 @@
+fun f(): String {
+    return "f()"
+}