Support for "Highest" Priority Obsidian Tasks in AI Planner

Please implement another top priority level to match the Obsidian Tasks Priority System.

AI Planner treats "high" and "highest" priority tasks imported from Obsidian Tasks as if they were the same priority level. Both use identical red dots as indicators and receive the same prioritization behaviour when populating the calendar, although in the sidebar they are sort properly when grouped by priority.