servoshell: Move headless setting to ServoShellPreferences (#35377)

This is only used in servoshell, even though it was plumbed through
script previously. It's just about how the `RenderingContext` is set up,
which is something managed entirely outside of servo itself.

In addition, make the name of `servo_shell_preferences` in `app.rs` more
consistent with the rest of the codebase.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/shared/script/lib.rs

@@ -770,8 +770,6 @@ pub struct WorkerGlobalScopeInit {
     pub origin: ImmutableOrigin,
     /// The creation URL
     pub creation_url: Option<ServoUrl>,
-    /// True if headless mode
-    pub is_headless: bool,
     /// An optional string allowing the user agnet to be set for testing.
     pub user_agent: Cow<'static, str>,
     /// True if secure context