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/collab_ui/src/notifications/collab_notification.rs

@@ -33,7 +33,7 @@ impl ParentElement for CollabNotification {
 
 impl RenderOnce for CollabNotification {
     fn render(self, cx: &mut WindowContext) -> impl IntoElement {
-        h_stack()
+        h_flex()
             .text_ui()
             .justify_between()
             .size_full()
@@ -42,9 +42,9 @@ impl RenderOnce for CollabNotification {
             .p_2()
             .gap_2()
             .child(img(self.avatar_uri).w_12().h_12().rounded_full())
-            .child(v_stack().overflow_hidden().children(self.children))
+            .child(v_flex().overflow_hidden().children(self.children))
             .child(
-                v_stack()
+                v_flex()
                     .child(self.accept_button)
                     .child(self.dismiss_button),
             )

crates/collab_ui/src/notifications/incoming_call_notification.rs

@@ -137,7 +137,7 @@ impl Render for IncomingCallNotification {
                     move |_, cx| state.respond(false, cx)
                 }),
             )
-            .child(v_stack().overflow_hidden().child(Label::new(format!(
+            .child(v_flex().overflow_hidden().child(Label::new(format!(
                 "{} is sharing a project in Zed",
                 self.state.call.calling_user.github_login
             )))),

crates/collab_ui/src/notifications/stories/collab_notification.rs

@@ -24,7 +24,7 @@ impl Render for CollabNotificationStory {
                         Button::new("decline", "Decline"),
                     )
                     .child(
-                        v_stack()
+                        v_flex()
                             .overflow_hidden()
                             .child(Label::new("maxdeviant is sharing a project in Zed")),
                     ),