Require `alt-tab` for `AcceptEditPrediction` when `tab` inserting whitespace is desired (#24705)

Moves tab whitespace insertion logic out of `AcceptEditPrediction`
handler.

`edit_prediction_requires_modifier` context will now be true when on a
line with leading whitespace, so that `alt-tab` is used to accept
predictions in this case. This way leading indentation can be typed when
edit predictions are visible

Release Notes:

- N/A

Co-authored-by: Ben <ben@zed.dev>
Co-authored-by: Joao <joao@zed.dev>