Closes https://github.com/zed-industries/zed/issues/25406
### Problem
Users have been confused about requiring `alt-tab` instead of just `tab`
in cases where they don't have a completions menu open (see issue
above). When they insert a newline and are in leading whitespace, they
expect to be able to accept a prediction with just `tab`, but doing so
increasing the indentation instead.
This PR changes the behavior in so a modifier is only required if the
cursor isn't already at the right indentation level based on the
surrounding block. In this case, `tab` would increase the indentation
and the prediction would get interpolated, allowing the user to press
`tab` again to accept it.
We also updated the docs to break down this behavior:
https://github.com/zed-industries/zed/pull/25493
### Before
https://github.com/user-attachments/assets/91fe6193-dddd-43c1-8c26-0f4648bdc3fa
### After
https://github.com/user-attachments/assets/671041bf-bf22-46a3-8466-b19b3e7dd6a0
Release Notes:
- edit predictions: Do not require a modifier key when indentation is
correct according to its surrounding block
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
