mode/document: Add scroll-half-page-{up,down} (C-d/C-u in vi-normal)

[danrasmuson]

Description

Add scroll-half-page-down and scroll-half-page-up commands and
bind them to C-d / C-u in the document-mode vi-normal keyscheme.

Vim users expect C-d / C-u to scroll by half a viewport. Nyxt
already ships scroll-page-{up,down} (bound to C-f / C-b,
space / shift-space, etc.) and scroll-to-{top,bottom} (gg /
G), but no half-page variant — so the vi-normal keyscheme
currently leaves C-d and C-u unbound and they fall through to
the renderer.

The new commands are tiny wrappers around
window.scrollBy(0, ±innerHeight/2), matching the style of the
existing scroll-page-{up,down}. They are bound alongside the
existing page-scroll bindings in vi-normal; cua and emacs
keyschemes are intentionally left unchanged since C-d / C-u
have meanings there (e.g. Emacs C-d = delete-char).

Fixes # (no existing issue — happy to file one if preferred)

Checklist:

• Git branch state is mergable.
• Changelog is up to date (via a separate commit). (n/a — I
  couldn't find a CHANGELOG file in the repo; happy to add a
  release-notes entry if you point me at the right location.)
• New dependencies are accounted for. (none)
• Documentation is up to date. (commands are self-documenting
  via their docstrings; no manual entries reference the existing
  scroll commands by name.)
• Compilation and tests ((asdf:test-system :nyxt/<renderer>))
  • No new compilation warnings.
  • Tests are sufficient. (no test changes; the new commands are
    one-liners structurally identical to the existing
    scroll-page-{up,down} and aren't covered by automated tests.)