Produce source maps for local typescriptServices build (#41589)

* Enable source maps for typescriptServices local builds

* Alias gulp lkg -> gulp LKG

Gulpfile.js

@@ -590,6 +590,7 @@ task("LKG").description = "Makes a new LKG out of the built js files";
 task("LKG").flags = {
     "   --built": "Compile using the built version of the compiler.",
 };
+task("lkg", series("LKG"));
 
 const generateSpec = () => exec("cscript", ["//nologo", "scripts/word2md.js", path.resolve("doc/TypeScript Language Specification - ARCHIVED.docx"), path.resolve("doc/spec-ARCHIVED.md")]);
 task("generate-spec", series(buildScripts, generateSpec));