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/workspace/src/dock.rs

@@ -9,7 +9,7 @@ use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use settings::SettingsStore;
 use std::sync::Arc;
-use ui::{h_stack, ContextMenu, IconButton, Tooltip};
+use ui::{h_flex, ContextMenu, IconButton, Tooltip};
 use ui::{prelude::*, right_click_menu};
 
 const RESIZE_HANDLE_SIZE: Pixels = Pixels(6.);
@@ -682,7 +682,7 @@ impl Render for PanelButtons {
                 )
             });
 
-        h_stack().gap_0p5().children(buttons)
+        h_flex().gap_0p5().children(buttons)
     }
 }