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/ui/src/components/stories/toggle_button.rs

@@ -25,7 +25,7 @@ impl Render for ToggleButtonStory {
             StorySection::new().child(
                 StoryItem::new(
                     "Toggle button group",
-                    h_stack()
+                    h_flex()
                         .child(
                             ToggleButton::new(1, "Apple")
                                 .style(ButtonStyle::Filled)
@@ -59,7 +59,7 @@ impl Render for ToggleButtonStory {
             StorySection::new().child(
                 StoryItem::new(
                     "Toggle button group with selection",
-                    h_stack()
+                    h_flex()
                         .child(
                             ToggleButton::new(1, "Apple")
                                 .style(ButtonStyle::Filled)