Closes https://github.com/zed-industries/zed/issues/22912

Reworks the task context infrastructure so that it's possible to have
multiple contexts at the same time, and stores all possible worktree
context there.
Task UI code is now falling back to the "active" worktree context, if
active item's context did not produce a resolved task.

Current code does not produce meaningful results for projects with
multiple worktrees to avoid ambiguity and design changes: instead of
resolving tasks per worktree context available, extra worktree context
is only used when resolving tasks from the same worktree.

Release Notes:

- Improved Zed tasks' `ZED_WORKTREE_ROOT` fallbacks