Add top-level CancellationToken support to Spectre.Console.Cli

[0xced]

Fixes #701

• I have read the Contribution Guidelines
• I have commented on the issue above and discussed the intended changes
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Opened as draft because the documentation still needs to be updated.

Changes

A new CancellationToken parameter has been added to the Run and RunAsync methods of ICommandApp and everywhere else needed.

This is a breaking change. Both Command and AsyncCommand subclasses will need to be updated to handle the new CancellationToken.

Before:

public sealed class MyCommand : AsyncCommand
{
    public override Task<int> ExecuteAsync(CommandContext context)
    {
        return Task.FromResult(0);
    }
}

After:

public sealed class MyCommand : AsyncCommand
{
    public override async Task<int> ExecuteAsync(CommandContext context, CancellationToken cancellationToken)
    {
        await Task.Delay(0, cancellationToken);
        return 0;
    }
}

When the application terminates because the cancellation token was triggered, the default exit code is 130. This value can be configured through the new CancellationExitCode() fluent method.

var app = new CommandApp();
app.Configure(config => config.CancellationExitCode(123));

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Please upvote 👍 this pull request if you are interested in it.

[0xced]

I will update the documentation in a follow-up pull request. Let's merge this to get it into a pre-release.

[patriksvensson]

LGTM 👍