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/ui/src/components/icon.rs

@@ -158,6 +158,7 @@ pub enum IconName {
     Exit,
     ExpandVertical,
     ExternalLink,
+    Eye,
     File,
     FileDoc,
     FileGeneric,
@@ -166,6 +167,8 @@ pub enum IconName {
     FileRust,
     FileToml,
     FileTree,
+    FileText,
+    FileCode,
     Filter,
     Folder,
     FolderOpen,
@@ -309,6 +312,7 @@ impl IconName {
             IconName::Exit => "icons/exit.svg",
             IconName::ExpandVertical => "icons/expand_vertical.svg",
             IconName::ExternalLink => "icons/external_link.svg",
+            IconName::Eye => "icons/eye.svg",
             IconName::File => "icons/file.svg",
             IconName::FileDoc => "icons/file_icons/book.svg",
             IconName::FileGeneric => "icons/file_icons/file.svg",
@@ -317,6 +321,8 @@ impl IconName {
             IconName::FileRust => "icons/file_icons/rust.svg",
             IconName::FileToml => "icons/file_icons/toml.svg",
             IconName::FileTree => "icons/project.svg",
+            IconName::FileCode => "icons/file_code.svg",
+            IconName::FileText => "icons/file_text.svg",
             IconName::Filter => "icons/filter.svg",
             IconName::Folder => "icons/file_icons/folder.svg",
             IconName::FolderOpen => "icons/file_icons/folder_open.svg",