Add store and help to SDK commands

I've added the store command as a localizable string (the
help command was already one), but I've not supplied any
translations.

Fixes #7432

test/dotnet-help.Tests/GivenThatIWantToShowHelpForDotnetCommand.cs

@@ -38,6 +38,8 @@ SDK commands:
   nuget            Provides additional NuGet commands.
   msbuild          Runs Microsoft Build Engine (MSBuild).
   vstest           Runs Microsoft Test Execution Command Line Tool.
+  store            Stores the specified assemblies in the runtime package store.
+  help             Show help.
 
 Common options:
   -v|--verbosity        Set the verbosity level of the command. Allowed values are q[uiet], m[inimal], n[ormal], d[etailed], and diag[nostic].