Generate relative paths in JS source maps

Also, add CLI options to manipulate prefixes of path

See KT-4078

compiler/testData/cli/js/sourceMapRootMissing.args

@@ -0,0 +1,10 @@
+$TESTDATA_DIR$/sourceMapRoot/foo/file1.kt
+$TESTDATA_DIR$/sourceMapRoot/bar/file2.kt
+$TESTDATA_DIR$/sourceMapRoot/bar/baz/file3.kt
+$TESTDATA_DIR$/sourceMapRoot/foo/baz/file4.kt
+-no-stdlib
+-source-map
+-source-map-source-roots
+$TESTDATA_DIR$/sourceMapRoot/foo
+-output
+$TEMP_DIR$/out.js