Introduce code generation (#2901)

![CleanShot 2023-08-28 at 12 24
36@2x](https://github.com/zed-industries/zed/assets/482957/f97cb399-1ac2-4fa9-94a7-137d1eec711c)


Release Notes:

- Added a new "Inline Assist" feature that lets you transform a
selection or generate new code at the cursor location by hitting
`ctrl-enter`.

crates/theme/src/theme.rs

@@ -1150,6 +1150,17 @@ pub struct AssistantStyle {
     pub api_key_editor: FieldEditor,
     pub api_key_prompt: ContainedText,
     pub saved_conversation: SavedConversation,
+    pub inline: InlineAssistantStyle,
+}
+
+#[derive(Clone, Deserialize, Default, JsonSchema)]
+pub struct InlineAssistantStyle {
+    #[serde(flatten)]
+    pub container: ContainerStyle,
+    pub editor: FieldEditor,
+    pub disabled_editor: FieldEditor,
+    pub pending_edit_background: Color,
+    pub include_conversation: ToggleIconButtonStyle,
 }
 
 #[derive(Clone, Deserialize, Default, JsonSchema)]