Generate relative paths in JS source maps

Also, add CLI options to manipulate prefixes of path

See KT-4078

compiler/testData/cli/js/sourceMapRoot/bar/baz/file3.kt

@@ -0,0 +1,3 @@
+fun h(): Long {
+    return 42L
+}

compiler/testData/cli/js/sourceMapRoot/bar/file2.kt

@@ -0,0 +1,3 @@
+fun g(): Int {
+    return 23
+}