Map additional text colors from Zed1 themes (#3796)

This PR maps additional text colors from the Zed1 themes to the Zed2
themes.

Release Notes:

- N/A

crates/editor2/src/element.rs

@@ -1724,7 +1724,7 @@ impl EditorElement {
         // Show the placeholder when the editor is empty
         if snapshot.is_empty() {
             let font_size = self.style.text.font_size.to_pixels(cx.rem_size());
-            let placeholder_color = cx.theme().styles.colors.text_placeholder;
+            let placeholder_color = cx.theme().colors().text_placeholder;
             let placeholder_text = snapshot.placeholder_text();
 
             let placeholder_lines = placeholder_text