Improve command completion.

This commit improves command completion by updating the `new` and `nuget` parsers to
match their current supported syntax.  Removes the unnecessary description
strings that were not used (these commands are parsed by assemblies external to
the CLI). The top level options are also sync'd to the currently supported
options.

Additionally, it unhides the `msbuild` and `vstest` commands so that `dotnet
complete` suggests them.

Fixes #9286.

test/dotnet.Tests/TelemetryCommandTest.cs

@@ -163,7 +163,7 @@ namespace Microsoft.DotNet.Tests
         {
             const string argumentToSend = "push";
 
-            string[] args = { "nuget", argumentToSend };
+            string[] args = { "nuget", argumentToSend, "path" };
 
             Cli.Program.ProcessArgs(args);
             _fakeTelemetry