Add preview tabs (#9125)

This PR implements the preview tabs feature from VSCode.
More details and thanks for the head start of the implementation here
#6782.

Here is what I have observed from using the vscode implementation ([x]
-> already implemented):
- [x] Single click on project file opens tab as preview
- [x] Double click on item in project panel opens tab as permanent
- [x] Double click on the tab makes it permanent
- [x] Navigating away from the tab makes the tab permanent and the new
tab is shown as preview (e.g. GoToReference)
- [x] Existing preview tab is reused when opening a new tab
- [x] Dragging tab to the same/another panel makes the tab permanent
- [x] Opening a tab from the file finder makes the tab permanent
- [x] Editing a preview tab will make the tab permanent
- [x] Using the space key in the project panel opens the tab as preview
- [x] Handle navigation history correctly (restore a preview tab as
preview as well)
- [x] Restore preview tabs after restarting
- [x] Support opening files from file finder in preview mode (vscode:
"Enable Preview From Quick Open")
 
I need to do some more testing of the vscode implementation, there might
be other behaviors/workflows which im not aware of that open an item as
preview/make them permanent.

Showcase:


https://github.com/zed-industries/zed/assets/53836821/9be16515-c740-4905-bea1-88871112ef86


TODOs
- [x] Provide `enable_preview_tabs` setting
- [x] Write some tests
- [x] How should we handle this in collaboration mode (have not tested
the behavior so far)
- [x] Keyboard driven usage (probably need workspace commands)
- [x] Register `TogglePreviewTab` only when setting enabled?
- [x] Render preview tabs in tab switcher as italic
- [x] Render preview tabs in image viewer as italic
- [x] Should this be enabled by default (it is the default behavior in
VSCode)?
- [x] Docs

Future improvements (out of scope for now):
- Support preview mode for find all references and possibly other
multibuffers (VSCode: "Enable Preview From Code Navigation")


Release Notes:

- Added preview tabs
([#4922](https://github.com/zed-industries/zed/issues/4922)).

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/gpui/src/styled.rs

@@ -1,7 +1,7 @@
 use crate::{
     self as gpui, hsla, point, px, relative, rems, AbsoluteLength, AlignItems, CursorStyle,
-    DefiniteLength, Fill, FlexDirection, FlexWrap, FontWeight, Hsla, JustifyContent, Length,
-    Position, SharedString, StyleRefinement, Visibility, WhiteSpace,
+    DefiniteLength, Fill, FlexDirection, FlexWrap, FontStyle, FontWeight, Hsla, JustifyContent,
+    Length, Position, SharedString, StyleRefinement, Visibility, WhiteSpace,
 };
 use crate::{BoxShadow, TextStyleRefinement};
 use smallvec::{smallvec, SmallVec};
@@ -681,6 +681,24 @@ pub trait Styled: Sized {
         self
     }
 
+    /// Set the font style to 'non-italic',
+    /// see the [Tailwind Docs](https://tailwindcss.com/docs/font-style#italicizing-text)
+    fn non_italic(mut self) -> Self {
+        self.text_style()
+            .get_or_insert_with(Default::default)
+            .font_style = Some(FontStyle::Normal);
+        self
+    }
+
+    /// Set the font style to 'italic',
+    /// see the [Tailwind Docs](https://tailwindcss.com/docs/font-style#italicizing-text)
+    fn italic(mut self) -> Self {
+        self.text_style()
+            .get_or_insert_with(Default::default)
+            .font_style = Some(FontStyle::Italic);
+        self
+    }
+
     /// Remove the text decoration on this element, this value cascades to its child elements.
     fn text_decoration_none(mut self) -> Self {
         self.text_style()