Introduce `TaskTemplate.unknown_variables`, which determines whether any
invalid variable name starting with `ZED_` is being used in the `label`,
`command`, `args`, `env` or `cwd` and, if that's the case, returns a
vector with all of the unique and invalid variable names.

Update `Inventory::update_file_based_tasks` in order to use
`TaskTemplate.unknown_variables` when parsing the templates, keeping
track of which ones have invalid variables, while keeping the valid ones
around. If any validation errors are found, an `Err` is returned, which
will trigger a validation error dialog to be shown.

This approach replaces the runtime validation, which showed errors when
opening the task picker, as I believe validating at save-time might be a
better UX, as users are immediately alerted about invalid variables
without having to open the task picker, and seems to be a simpler
solution as we don't need a new subscription, new event and we avoid
upgrading a `cx.background_spawn` to a `cx.spawn`, which runs in the
main thread so it can block the UI thread, so we can continue keeping
the task resolution off the UI thread.

- Introduce `project::ToastLink` struct
- Add optional `link` field to `project::Event::Toast`, allowing toasts
  to include a button that opens a URL
- Update the toast shown when the local tasks parsing fails, to include
  a link to Zed's Task Variables documentation