Add duplicate-line and scroll-without-moving-cursor shortcuts

[NisargJasani0602]

Summary

Follow-up to #4222 — adds two of the "easy" tier bindings identified
in the Sublime keymap gap analysis there, per @khanniie's suggestion
to tackle that tier next.

• Shift-Ctrl-D / Shift-Cmd-D: duplicate the current line, using
  CodeMirror's existing copyLineDown command.
• Ctrl-Up/Down (Ctrl-Alt-Up/Down on Mac, matching Sublime's actual
  Mac binding): scroll the viewport by one line without moving the
  cursor.

Ctrl-J (join lines) is intentionally left out — it has no
CodeMirror 6 built-in and would need custom logic, so deferring it to
a smaller, separate follow-up rather than scope-creeping this PR.

Note: this PR only adds the keybindings themselves. Updating
KeyboardShortcutModal.jsx to document them (along with the broken
Replace shortcut and #4222's move-line shortcut) is tracked separately
in #4253.

Test plan

• npm run lint passes
• npm test -- client/modules/IDE/components/Editor passes
• Manually verified in the running dev server: Shift-Ctrl-D
  duplicates the current line; Ctrl-Up/Down scrolls the viewport
  without moving the cursor/selection

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines