Rename h_stack and v_stack to h_flex and v_flex, respectively (#4053)

This PR renames the `h_stack` and `v_stack` to `h_flex` and `v_flex`,
respectively.

We were previously using `h_stack` and `v_stack` to match SwiftUI, but
`h_flex` and `v_flex` fit better with the web/flexbox terminology that
the rest of GPUI uses.

Additionally, we were already calling the utility functions used to
implement `h_stack` and `v_stack` by the new names.

Release Notes:

- N/A

crates/recent_projects/src/recent_projects.rs

@@ -104,7 +104,7 @@ impl FocusableView for RecentProjects {
 
 impl Render for RecentProjects {
     fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
-        v_stack()
+        v_flex()
             .w(rems(self.rem_width))
             .child(self.picker.clone())
             .on_mouse_down_out(cx.listener(|this, _, cx| {
@@ -236,7 +236,7 @@ impl PickerDelegate for RecentProjectsDelegate {
                 .spacing(ListItemSpacing::Sparse)
                 .selected(selected)
                 .child(
-                    v_stack()
+                    v_flex()
                         .child(highlighted_location.names)
                         .when(self.render_paths, |this| {
                             this.children(highlighted_location.paths)