This changes the behaviour of `<tab>` when inline completion is visible.
When the cursor is before the suggested indentation level, accepting a
completion should just indent.

cc @nathansobo @maxdeviant 

Release Notes:

- Changed the behavior of `<tab>` at start of line when an inline
completion (Copilot, Supermaven, ...) is visible. If the cursor is
before the suggested indentation, `<tab>` now indents the line instead
of accepting the visible completion.

Co-authored-by: Antonio <antonio@zed.dev>