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/diagnostics/src/items.rs

@@ -6,7 +6,7 @@ use gpui::{
 };
 use language::Diagnostic;
 use lsp::LanguageServerId;
-use ui::{h_stack, prelude::*, Button, ButtonLike, Color, Icon, IconName, Label, Tooltip};
+use ui::{h_flex, prelude::*, Button, ButtonLike, Color, Icon, IconName, Label, Tooltip};
 use workspace::{item::ItemHandle, StatusItemView, ToolbarItemEvent, Workspace};
 
 use crate::{Deploy, ProjectDiagnosticsEditor};
@@ -23,14 +23,14 @@ pub struct DiagnosticIndicator {
 impl Render for DiagnosticIndicator {
     fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
         let diagnostic_indicator = match (self.summary.error_count, self.summary.warning_count) {
-            (0, 0) => h_stack().map(|this| {
+            (0, 0) => h_flex().map(|this| {
                 this.child(
                     Icon::new(IconName::Check)
                         .size(IconSize::Small)
                         .color(Color::Default),
                 )
             }),
-            (0, warning_count) => h_stack()
+            (0, warning_count) => h_flex()
                 .gap_1()
                 .child(
                     Icon::new(IconName::ExclamationTriangle)
@@ -38,7 +38,7 @@ impl Render for DiagnosticIndicator {
                         .color(Color::Warning),
                 )
                 .child(Label::new(warning_count.to_string()).size(LabelSize::Small)),
-            (error_count, 0) => h_stack()
+            (error_count, 0) => h_flex()
                 .gap_1()
                 .child(
                     Icon::new(IconName::XCircle)
@@ -46,7 +46,7 @@ impl Render for DiagnosticIndicator {
                         .color(Color::Error),
                 )
                 .child(Label::new(error_count.to_string()).size(LabelSize::Small)),
-            (error_count, warning_count) => h_stack()
+            (error_count, warning_count) => h_flex()
                 .gap_1()
                 .child(
                     Icon::new(IconName::XCircle)
@@ -64,7 +64,7 @@ impl Render for DiagnosticIndicator {
 
         let status = if !self.in_progress_checks.is_empty() {
             Some(
-                h_stack()
+                h_flex()
                     .gap_2()
                     .child(Icon::new(IconName::ArrowCircle).size(IconSize::Small))
                     .child(
@@ -91,7 +91,7 @@ impl Render for DiagnosticIndicator {
             None
         };
 
-        h_stack()
+        h_flex()
             .h(rems(1.375))
             .gap_2()
             .child(