feat(tui): bind to toggle task selection pinning (#9609)
### Description
While task selection pinning is convenient when tasks are changing order
and status, it can sometimes get in the way.
When you haven't selected a task yet (e.g. pressed one of ↑/↓/j/k), you
are in an _unpinned_ state. You'll always be looking at the first task
in the list, even as they shuffle around in order. When you do press one
of those keys, you end up in a _pinned_ state such that the task you
selected continues to be selected.
This feels good until you select a task, and then want to "exit" that
pinned state and let tasks flow around beneath your cursor again. In
this PR, we solve this problem, adding a keybind for `p` that toggles
the pinned/unpinned state of your selection.
### Testing Instructions
Pick out your favorite Turborepo, run tasks with the TUI, and try out
this new feature.
