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/terminal_view/src/terminal_panel.rs

@@ -13,7 +13,7 @@ use search::{buffer_search::DivRegistrar, BufferSearchBar};
 use serde::{Deserialize, Serialize};
 use settings::Settings;
 use terminal::terminal_settings::{TerminalDockPosition, TerminalSettings};
-use ui::{h_stack, ButtonCommon, Clickable, IconButton, IconSize, Selectable, Tooltip};
+use ui::{h_flex, ButtonCommon, Clickable, IconButton, IconSize, Selectable, Tooltip};
 use util::{ResultExt, TryFutureExt};
 use workspace::{
     dock::{DockPosition, Panel, PanelEvent},
@@ -68,7 +68,7 @@ impl TerminalPanel {
             pane.display_nav_history_buttons(false);
             pane.set_render_tab_bar_buttons(cx, move |pane, cx| {
                 let terminal_panel = terminal_panel.clone();
-                h_stack()
+                h_flex()
                     .gap_2()
                     .child(
                         IconButton::new("plus", IconName::Plus)