gpui: Add scroll anchors (#19894)

## Problem statement
I want to add keyboard navigation support to SSH modal. Doing so is
possible in current landscape, but not particularly ergonomic;
`gpui::ScrollHandle` has `scroll_to_item` API that takes an index of the
item you want to scroll to. The problem is, however, that it only works
with it's immediate children - thus in order to support scrolling via
keyboard you have to bend your UI to have a particular layout. Even when
your list of items is perfectly flat, having decorations inbetween items
is problematic as they are also children of the list, which means that
you either have to maintain the mapping to devise a correct index of an
item that you want to scroll to, or you have to make the decoration a
part of the list item itself, which might render the scrolling imprecise
(you might e.g. not want to scroll to a header, but to a button beneath
it).

## The solution
This PR adds `ScrollAnchor`, a new kind of handle to the gpui. It has a
similar role to that of a ScrollHandle, but instead of tracking how far
along an item has been scrolled, it tracks position of an element
relative to the parent to which a given scroll handle belongs. In short,
it allows us to persist the position of an element in a list of items
and scroll to it even if it's not an immediate children of a container
whose scroll position is tracked via an associated scroll handle.
Additionally this PR adds a new kind of the container to the UI crate
that serves as a convenience wrapper for using ScrollAnchors. This
container provides handlers for `menu::SelectNext` and
`menu::SelectPrev` and figures out which item should be focused next.

Release Notes:

- Improve keyboard navigation in ssh modal

crates/recent_projects/src/ssh_connections.rs

@@ -63,7 +63,7 @@ impl SshSettings {
     }
 }
 
-#[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
+#[derive(Clone, Default, Serialize, Deserialize, PartialEq, JsonSchema)]
 pub struct SshConnection {
     pub host: SharedString,
     #[serde(skip_serializing_if = "Option::is_none")]
@@ -100,7 +100,7 @@ impl From<SshConnection> for SshConnectionOptions {
     }
 }
 
-#[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
+#[derive(Clone, Default, Serialize, PartialEq, Deserialize, JsonSchema)]
 pub struct SshProject {
     pub paths: Vec<String>,
 }