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/stack.rs

@@ -4,12 +4,12 @@ use crate::StyledExt;
 
 /// Horizontally stacks elements. Sets `flex()`, `flex_row()`, `items_center()`
 #[track_caller]
-pub fn h_stack() -> Div {
+pub fn h_flex() -> Div {
     div().h_flex()
 }
 
 /// Vertically stacks elements. Sets `flex()`, `flex_col()`
 #[track_caller]
-pub fn v_stack() -> Div {
+pub fn v_flex() -> Div {
     div().v_flex()
 }