Fix spacing around hidden status bar items (#39992)

This is a follow-up PR to
https://github.com/zed-industries/zed/pull/39609, and attempts to
address hidden status bar items still contributing to the layout and
creating extra spacing.

![before using display:none theres extra spaces, afterwords the buttons
are always evenly
spaced](https://github.com/user-attachments/assets/3bd07837-5f6f-4ca1-8985-9f3cb8b6893d)

- 203cbd634bfb1489b8afa4952d9594615a956b77 Adds a `.none()` method to
the `gpui::Styled` helper trait, so that status items can set their
display type to none inside their `render` method.

- 249f06e3de63b0ab32814f20e7105d8e2b642f02 Applies `.none()` to all the
status items.

- ~~499f564906c88336608c81615b11ebc9ab43d832~~ At first I was adding an
`is_visible` method to the `StatusBarView` trait, which would be used to
skip status bar items which would just render an empty div anyway, but I
felt duplicating the conditions for hiding the buttons between the
status items `is_visible` and `render` methods could be an attraction
for bugs, so I tried to find another approach. This commit contains
those changes, reverted immediately (if the `is_visible` approach is
preferred I can bring it back!)

- f37cb75f0519ceea1f3e1cc4f97087a5cb34b0fd (bonus!) Adds a condition to
the vim mode indicator to avoid a leading space when there are no
pending keys.

Release Notes:

- N/A

crates/diagnostics/src/items.rs

@@ -30,7 +30,7 @@ impl Render for DiagnosticIndicator {
     fn render(&mut self, _: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
         let indicator = h_flex().gap_2();
         if !ProjectSettings::get_global(cx).diagnostics.button {
-            return indicator;
+            return indicator.hidden();
         }
 
         let diagnostic_indicator = match (self.summary.error_count, self.summary.warning_count) {