[commands] Mark CommandPtr class as [[nodiscard]] (#7803)

This has the same effect but makes it so any user code returning CommandPtr can't discard a returned command.

Signed-off-by: Eric Ward <ezeward4@gmail.com>

wpilibNewCommands/src/test/native/cpp/frc2/command/CommandPtrTest.cpp

@@ -38,5 +38,6 @@ TEST_F(CommandPtrTest, MovedFrom) {
 }
 
 TEST_F(CommandPtrTest, NullInitialization) {
-  EXPECT_THROW(CommandPtr{std::unique_ptr<Command>{}}, frc::RuntimeError);
+  EXPECT_THROW(auto cmd = CommandPtr{std::unique_ptr<Command>{}},
+               frc::RuntimeError);
 }