--emitDeclarationsOnly flag to enable declarations only output (#20735)

* Add emitOnlyDeclarations flag

* Fix name

* verifyOptions checking logic

* Passing tests

* doJsEmitBaseline

* Tests !!!

src/compiler/commandLineParser.ts

@@ -186,6 +186,12 @@ namespace ts {
             category: Diagnostics.Basic_Options,
             description: Diagnostics.Generates_corresponding_d_ts_file,
         },
+        {
+            name: "emitDeclarationsOnly",
+            type: "boolean",
+            category: Diagnostics.Advanced_Options,
+            description: Diagnostics.Only_emit_d_ts_declaration_files,
+        },
         {
             name: "sourceMap",
             type: "boolean",