Page up/down and half up/down consistency

[EpocSquadron]

If it intent is to be consistent with the keybindings for screen movement from vim, we should ensure they are in place and act consistently throughout:

• [x] Add ctrl-f and ctrl-b as equivalents of page down and page up in normal mode
• [ ] Implement ctrl-d and ctrl-u for popups
• [ ] Implement ctrl-d and ctrl-u for pickers
• [x] Add ctrl-f and ctrl-b to popups, to complement the half height equivalents

[sudormrfbin]

• Add ctrl-f and ctrl-d as equivalents of page down and page up in normal mode

We already have ctrl-f, ctrl-b, ctrl-d and ctrl-u in normal mode:

https://github.com/helix-editor/helix/blob/392dfa0841fb106300eedf26f8628e48a32d0ea4/helix-term/src/keymap.rs#L628-L631

[EpocSquadron]

Somehow missed that.

[EpocSquadron]

#1612 will fix the third task, and leave the half-height equivalents for another PR.

[EpocSquadron]

Updated for accuracy. We just have a few places to implement the half-page movements and then this is all set.

[Rudxain]

Is there a chance that some future release will have a j/k equivalent for doc popups? I like the half-page movement, but sometimes it's convenient to have fine-grained per-line movement