feat: add more vim movement bindings for navigation (#3041)

Add vim-style movement commands to vim_normal keymap mode:

- Ctrl+u/d: half-page up/down
- Ctrl+b/f: full-page up/down
- G: jump to visual bottom of history
- gg: jump to visual top of history (multi-key sequence)
- H/M/L: jump to top/middle/bottom of visible screen

Also adds `offset()` getter to ListState for H/M/L functionality, and
includes tests for the new bindings.

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## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
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- [x] I have checked that there are no existing pull requests for the
same thing

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

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