Validate task variable names when the file is saved, immediately
displaying an error toast if any invalid `ZED_*` variables are found.
Valid tasks in the same file are still loaded and work normally.

- Add `task::task_template::TaskTemplate::unknown_variables()` to detect
  invalid `ZED_` variable names
- Add `project::ToastLink` struct and optional `link` field to
  `project::Event::Toast`
- Show documentation link in the error toast

Closes #23275

Release Notes:

- Fixed user-defined tasks with unresolved `ZED_*` variables being
silently omitted

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Co-authored-by: dino <dinojoaocosta@gmail.com>