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/prelude.rs

@@ -13,7 +13,7 @@ pub use crate::fixed::*;
 pub use crate::selectable::*;
 pub use crate::styles::{vh, vw};
 pub use crate::visible_on_hover::*;
-pub use crate::{h_stack, v_stack};
+pub use crate::{h_flex, v_flex};
 pub use crate::{Button, ButtonSize, ButtonStyle, IconButton, SelectableButton};
 pub use crate::{ButtonCommon, Color, StyledExt};
 pub use crate::{Headline, HeadlineSize};