Add types to the E2E style guide
What does this MR do?
Adds a few types that cover elements that aren't covered by existing types.
-
_placeholder
refers to a temporary element that appears while content is loading. For example, the elements that are displayed instead of discussions while the discussions are being fetched. -
_container
refers to an element that includes other elements, but doesn't present visible content itself. E.g., an element that has a third-party editor inside it, but which isn't the editor itself and so doesn't include the editor's content. -
_modal
refers to a popup modal dialog, e.g., a confirmation prompt.