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/button/button.rs

@@ -362,7 +362,7 @@ impl RenderOnce for Button {
         };
 
         self.base.child(
-            h_stack()
+            h_flex()
                 .gap_1()
                 .when(self.icon_position == Some(IconPosition::Start), |this| {
                     this.children(self.icon.map(|icon| {
@@ -375,7 +375,7 @@ impl RenderOnce for Button {
                     }))
                 })
                 .child(
-                    h_stack()
+                    h_flex()
                         .gap_2()
                         .justify_between()
                         .child(