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/activity_indicator/src/activity_indicator.rs

@@ -295,7 +295,7 @@ impl Render for ActivityIndicator {
     fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
         let content = self.content_to_render(cx);
 
-        let mut result = h_stack()
+        let mut result = h_flex()
             .id("activity-indicator")
             .on_action(cx.listener(Self::show_error_message))
             .on_action(cx.listener(Self::dismiss_error_message));