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/storybook/src/stories/overflow_scroll.rs

@@ -11,7 +11,7 @@ impl Render for OverflowScrollStory {
             .child(Story::title("Overflow Scroll"))
             .child(Story::label("`overflow_x_scroll`"))
             .child(
-                h_stack()
+                h_flex()
                     .id("overflow_x_scroll")
                     .gap_2()
                     .overflow_x_scroll()
@@ -24,7 +24,7 @@ impl Render for OverflowScrollStory {
             )
             .child(Story::label("`overflow_y_scroll`"))
             .child(
-                v_stack()
+                v_flex()
                     .id("overflow_y_scroll")
                     .gap_2()
                     .overflow_y_scroll()

crates/storybook/src/stories/text.rs

@@ -117,7 +117,7 @@ impl Render for TextStory {
 //     type Element = Div;
 
 //     fn render(&mut self, cx: &mut gpui::ViewContext<Self>) -> Self::Element {
-//         v_stack()
+//         v_flex()
 //             .bg(blue())
 //             .child(
 //                 div()