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/checkbox.rs

@@ -103,7 +103,7 @@ impl RenderOnce for Checkbox {
             ),
         };
 
-        h_stack()
+        h_flex()
             .id(self.id)
             // Rather than adding `px_1()` to add some space around the checkbox,
             // we use a larger parent element to create a slightly larger