feat: emit TextAction::Cancelled on Escape key press and expose on_escape callback for TextInput

[phamsongnewl]

Summary

• Add TextAction::Cancelled variant to TextAction enum, emitted when the Escape key is pressed in a TextArea.
• Add on_escape callback builder to TextInput (xilem), mirroring the existing on_enter pattern.
• Handle TextAction::Cancelled in TextInput's message dispatch to invoke the registered callback.

Changes

masonry/src/widgets/text_area.rs

• New TextAction::Cancelled variant with doc comment.
• Key::Named(NamedKey::Escape) handling that submits TextAction::Cancelled via ctx.submit_action.
  xilem_masonry/src/view/text_input.rs
• New on_escape: Option<Callback<State, Action>> field.
• TextInput::on_escape(F) builder method (matches on_enter signature).
• TextAction::Cancelled handling in View::message — calls the callback or returns Stale.

Motivation

Escape is a universal cancel/dismiss gesture in desktop UIs. TextArea previously had no way to surface it to the view layer, forcing users to work around it at the event level.