fix https://github.com/brave/brave-browser/issues/20500
Applied bookmark part of chromium's side panel to sidebar bookmark panel.
sidebar.mojom is used for interacting between sidebar's bookmarks panel
with SidebarBookmarksPageHandler.
Most of webui impls in browser/resources/sidebar/bookmarks are copied from upstream
and modified to show bookmark part only.
(side panel's page shows read later and bookmark part both.)
Bookmarks page uses custom context menu by asking to native via
Sidebar:ShowCustomContextMenu interface.
mkarolin