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/quick_action_bar/src/quick_action_bar.rs

@@ -93,7 +93,7 @@ impl Render for QuickActionBar {
             },
         );
 
-        h_stack()
+        h_flex()
             .id("quick action bar")
             .gap_2()
             .children(inlay_hints_button)