Add support for OSC777 (Send Notification)

[carlos-zamora]

Summary of the Pull Request

targets #20010

This adds support for the OSC 777 ; notify ; title ; body ST sequence. This allows client applications to send a notification to the Terminal. When this notification is clicked, it summons the terminal window that sent it.

Validation Steps Performed

# in PowerShell. Terminal should not be focused.
sleep 2; Write-Output "`e]777;notify;Hello;This is a notification`a"

PR Checklist

• Closes Send Desktop Notification via VT Sequence (OSC777) #7718
• Documentation updated
• Schema updated (if necessary)

Heavily based on #19938
Co-authored by @zadjii-msft

[lhecker]

After #20009 merged yesterday, I think this now needs an:

else
{
    _api.UnknownSequence();
}

[carlos-zamora]

Good catch. I never would've caught this haha

[carlos-zamora]

Do we need another one of those here?

terminal/src/terminal/adapter/adaptDispatch.cpp

Lines 3854 to 3857 in 34b985d

	if (!_optionalFeatures.test(OptionalFeature::DesktopNotification))
	{
	return;
	}

[lhecker]

Another one? What do you mean? But we should probably move the check down into the if (action == L"notify") block. That of course means that there's some redundant work we do if it's disabled, but... that'd only matter if we process >100s of these per second and I don't think we will.

[lhecker]

FWIW, you can drop those braces here.