[Reader Mode]: Prevent default anchor navigation before handling reading mode link

Updated the onclick handler for <a> elements to accept a MouseEvent and
call event.preventDefault(). This avoids the browser navigating to the
href when clicked, allowing chrome.readingMode.onLinkClicked(nodeId) to
handle the click exclusively.

Fixed: 409349223
Change-Id: Ia6791ab76dc3afd254f1915d1ad899fad701dddb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6664740
Reviewed-by: Lauren Winston <lwinston@google.com>
Reviewed-by: Kristi Saney <kristislee@google.com>
Commit-Queue: Jacobo Misael Tapia <jatapiaro@google.com>
Cr-Commit-Position: refs/heads/main@{#1478269}