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/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
             )))),