terminal: Make alternate_scroll on by default (#27302)

Most terminal emulators, like macOS Terminal, Alacritty, and Ghostty,
have alternate scroll turned on by default. I think it makes sense for
the Zed terminal to do the same and make it more of an opt-out feature.

Release Notes:

- N/A

crates/terminal/src/terminal_settings.rs

@@ -181,7 +181,7 @@ pub struct TerminalSettingsContent {
     /// presses when in the alternate screen (e.g. when running applications
     /// like vim or  less). The terminal can still set and unset this mode.
     ///
-    /// Default: off
+    /// Default: on
     pub alternate_scroll: Option<AlternateScroll>,
     /// Sets whether the option key behaves as the meta key.
     ///