Reuse menu::Item trait in picker (#2814)

* Refactor menu::Item to accomodate external state

Will be useful for storing editor state when reused by pickers.

* Add some type aliases for readability

* Reuse menu::Item trait in picker

This opens the way for merging the menu and picker code in the
future, since a picker is essentially a menu + prompt. More
excitingly, this change will also allow aligning items in the
picker, which would be useful (for example) in the command palette
for aligning the descriptions to the left and the keybinds to
the right in two separate columns.

The item formatting of each picker has been kept as is, even though
there is room for improvement now that we can format the data into
columns, since that is better tackled in a separate PR.

* Rename menu::Item::EditorData to Data

* Call and inline filter_text() in sort_text() completion

* Rename diagnostic picker's Item::Data

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