Add support for scrolling w/ mouse when explicitly enabled (#389)

Co-authored-by: Ulyssa <git@ulyssa.dev>

src/tests.rs

@@ -189,6 +189,7 @@ pub fn mock_tunables() -> TunableValues {
         external_edit_file_suffix: String::from(".md"),
         username_display: UserDisplayStyle::Username,
         message_user_color: false,
+        mouse: Default::default(),
         notifications: Notifications {
             enabled: false,
             via: NotifyVia::default(),