Consider h/l keys to shrink or grow the split

[kevinswiber]

Owner idea from #411 live-testing: h and l as keyboard commands to shrink/grow the master
pane, complementing the divider's focused ArrowLeft/Right. Today keyboard resize requires reaching
the divider (Tab, or click-to-focus); h/l would make the ratio adjustable from anywhere, the
way j/k step the selection from anywhere.

Design considerations before implementing:

• h/l complete the vim cluster next to j/k, which reads naturally as "left/right" — but in
  vim-land they usually mean cursor movement, and here they'd mean layout movement; confirm
  that reading is acceptable.
• Step size and floors should reuse the divider controller's math (split.ts) and write through
  the same single writer (prefs.applySplit), so the pref, the grid, and aria-valuenow can't
  disagree.
• Interaction with reading mode: should h past the floor snap into reading mode like the
  divider's ArrowLeft does, and should l from reading mode restore the split?
• The keys are currently unbound outside typing targets, so no conflicts — but the help sheet and
  the command palette should both surface them if added.