Tokenizing of the Options adds other options as arguments

Seems that if you miss the Argument for an Option, then command-line-api with parse the next Option as the Argument. This doesn't seem to me like expected behaviour. If this is expected behaviour, is there a way to alter the parser, or plugin parser behaviour?

I have the following code:
```
static async Task Main(string[] args)
{
var fileOption = new Option("--file", "file to load") {Arity = new ArgumentArity(1, 1)};
var sourceUrlOption = new Option("--source", "source url") { Arity = new ArgumentArity(1, 1) };
var load = new Command("load") { fileOption, sourceUrlOption };
load.Handler = CommandHandler.Create(DoSomethingAsync);
var rootCommand = new RootCommand {load};

await rootCommand.InvokeAsync(args);
}

static async Task DoSomethingAsync(string file, string source)
{
Console.WriteLine($"{file} {source}");
await Task.Delay(100);
}
```
When called using the following:
```
$ dotnet my-commandline load --file --source
```
I would expect the following error output from command-line-api:
```
Required argument missing for option: '--file'.
Required argument missing for option: '--source'.
...
```
However, the application runs and Invokes the DoSomething Async method with the file variable being set to the string '--source'. Outputing:
```
--source
```
It seems that the issue is in [StringExtensions.cs](https://github.com/dotnet/command-line-api/blob/main/src/System.CommandLine/Parsing/StringExtensions.cs) in the Tokenize method at line 131. It adds the next Option as an Argument to the previous Option. I would have thought that the expected behavour would be to add an Option. Which would then result in both the Options being added to the tokenList object.