skip mode: implement task modifier

[oliver-sanders]

Implement a task modifier for tasks that have been configured to run in skip mode.

The configured run mode should be made available as a task attribute so that the UI's can display skip/simulation mode tasks with a task modifier (the held and runahead "badges" are task modifiers).

The order of precedence for this modifier should be bellow the held and runahead states, but above queued.

--- https://github.com/cylc/cylc-admin/blob/master/docs/proposal-skip-mode.md#proposal-skip-run-mode-for-tasks

Perhaps an icon something along the lines of: https://icon-sets.iconify.design/carbon/skip-forward-outline-solid/

We should get this information into the task description that we display in the context menu. For example, a queued task may appear with the text "waiting (queued)". Perhaps it should say "waiting (configured to skip)" for skip mode tasks.

TODO:

• [ ] Add modifier.
• [ ] Ensure the modifier is listed on the "UI Quickstart" page.
• [ ] Add this modifier into the task icon component tests (this will generate a screenshot of it when the tests run).
• [ ] Copy this into the task/job states section in cylc-doc.
• [ ] Add task description into context menu.