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_view.rs

@@ -20,7 +20,7 @@ use terminal::{
     Clear, Copy, Event, MaybeNavigationTarget, Paste, ShowCharacterPalette, Terminal,
 };
 use terminal_element::TerminalElement;
-use ui::{h_stack, prelude::*, ContextMenu, Icon, IconName, Label};
+use ui::{h_flex, prelude::*, ContextMenu, Icon, IconName, Label};
 use util::{paths::PathLikeWithPosition, ResultExt};
 use workspace::{
     item::{BreadcrumbText, Item, ItemEvent},
@@ -697,7 +697,7 @@ impl Item for TerminalView {
         cx: &WindowContext,
     ) -> AnyElement {
         let title = self.terminal().read(cx).title(true);
-        h_stack()
+        h_flex()
             .gap_2()
             .child(Icon::new(IconName::Terminal))
             .child(Label::new(title).color(if selected {