Add shift and modifier key handling again

[reakaleek]

Context

This was unintentionally removed in #721

[Copilot]

Pull Request Overview

This PR reintroduces shift and modifier key handling for opening links in a new window using htmx events.

• Adds a listener for the htmx:beforeRequest event.
• Implements OS-based modifier key checks to decide when to open links in a new tab.

Comments suppressed due to low confidence (1)

src/Elastic.Markdown/Assets/main.ts:19

• Consider verifying that UAParser consistently returns 'macOS' for macOS devices. If it returns a different format (e.g., 'Mac OS'), the modifier key check might fail.

const modifierKey: boolean = os === 'macOS' ? metaKey : ctrlKey;