use separate icon element for menu item icons

The previous approach applied codicon CSS classes directly to the
action-label element, causing label text to render in the codicon
font. Instead, create a separate menu-item-icon span between the
check and label. When checked, the icon hides and the check shows.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>