Add toast notification infrastructure (#20010)

## Summary of the Pull Request
Adds the infrastructure for toast notifications. Breakdown:
- `DesktopNotification`:
- `DesktopNotificationArgs` includes the struct to group all
notification-related data together.
   - `SendNotification()` actually sends it
- `AppCommandlineArgs.cpp`: added a check for the `--from-toast` no-op
sentinel; ensures no new window is created
- Most of the other changes are just bubbling up the notification from
the `TerminalPaneContent` to `TerminalPage`
- `TabManagement.cpp`: `_SendDesktopNotification()` does the final
packaging of the notification before calling the `DesktopNotification`
API

This supports finding the right tab when it's been reordered or even
moved to a new window!
This also has expanded to support finding the right pane, which is
resilient to pane swaps/closing too. When the pane can't be found, we
just fallback to the tab.
If the pane is already focused, we don't send a notification.

This simply adds the infrastructure! Looks like nothing can actually
take advantage of it yet, but it's been tested with the changes in
#20011.

Heavily based on #19935
Co-authored by @zadjii-msft