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/list/list.rs

@@ -1,7 +1,7 @@
 use gpui::AnyElement;
 use smallvec::SmallVec;
 
-use crate::{prelude::*, v_stack, Label, ListHeader};
+use crate::{prelude::*, v_flex, Label, ListHeader};
 
 #[derive(IntoElement)]
 pub struct List {
@@ -47,7 +47,7 @@ impl ParentElement for List {
 
 impl RenderOnce for List {
     fn render(self, _cx: &mut WindowContext) -> impl IntoElement {
-        v_stack().w_full().py_1().children(self.header).map(|this| {
+        v_flex().w_full().py_1().children(self.header).map(|this| {
             match (self.children.is_empty(), self.toggle) {
                 (false, _) => this.children(self.children),
                 (true, Some(false)) => this,