Add -keep command line option to JS DCE tool

compiler/testData/cli/js-dce/dceHelp.out

@@ -1,6 +1,8 @@
 Usage: kotlin-dce-js <options> <source files>
 where possible options include:
   -output-dir <path>         Output directory
+  -keep <fully.qualified.name[,]>
+                             List of fully-qualified names of declarations that shouldn't be eliminated
   -help (-h)                 Print a synopsis of standard options
   -X                         Print a synopsis of advanced options
   -version                   Display compiler version

compiler/testData/cli/js-dce/include.js

@@ -0,0 +1,20 @@
+define(["exports"], function(exports) {
+    function foo() {
+    }
+
+    function bar() {
+    }
+
+    function baz() {
+    }
+
+    function ignore() {
+    }
+
+    baz();
+
+    exports.foo = foo;
+    exports.bar = bar;
+    exports.baz = baz;
+    exports.ignore = ignore;
+});

compiler/testData/cli/js-dce/includeDeclarations.args

@@ -0,0 +1,6 @@
+$TESTDATA_DIR$/include.js
+-output-dir
+$TEMP_DIR$/min
+-keep
+global.foo,global.bar
+-Xprint-reachability-info