Add shortcut to jump to next result file

[lewbei]

Fixes #34.

This adds the requested keyboard shortcut for moving from the current search result to the first result in the next file group.

Behavior:

• plain ArrowDown still moves to the next match as before
• Ctrl+ArrowDown jumps to the first match whose file differs from the currently selected match
• Cmd+ArrowDown is also supported for macOS keyboards
• if no match is selected yet, the shortcut selects the first match

Verification:

npm run compile

npm run compile passes.

I also ran npm run lint, but the repository currently has no ESLint config file, so ESLint exits before checking source files:

ESLint couldn't find a configuration file.