Add Markdown Preview Toggle (#15215)

Add a "Preview Markdown" button to the quick action bar when in a
markdown editor.

While it isn't my favorite, I went with the basic eye icon to be a bit
more generic so we can extend this control to allow opening other
previews such as SVGs like @jansol mentioned.

![CleanShot 2024-07-26 at 11 02
16@2x](https://github.com/user-attachments/assets/415963ce-d19e-432d-b8c2-37e7c6e52683)


https://github.com/user-attachments/assets/5980272c-eab9-4f69-86b6-0c593c25b525

---

Release Notes:

- Added a button to preview Markdown files in the toolbar.
`Option|Alt+Click` will open the preview to the side.

crates/markdown_preview/src/markdown_preview_view.rs

@@ -99,7 +99,7 @@ impl MarkdownPreviewView {
             .and_then(|view| pane.index_for_item(&view))
     }
 
-    fn resolve_active_item_as_markdown_editor(
+    pub fn resolve_active_item_as_markdown_editor(
         workspace: &Workspace,
         cx: &mut ViewContext<Workspace>,
     ) -> Option<View<Editor>> {
@@ -278,7 +278,7 @@ impl MarkdownPreviewView {
         }
     }
 
-    fn is_markdown_file<V>(editor: &View<Editor>, cx: &mut ViewContext<V>) -> bool {
+    pub fn is_markdown_file<V>(editor: &View<Editor>, cx: &mut ViewContext<V>) -> bool {
         let language = editor.read(cx).buffer().read(cx).language_at(0, cx);
         language
             .map(|l| l.name().as_ref() == "Markdown")