Split off various modules into "editing" folder. (#440)

Beside code moves, the only changes in this PR are:
- Changed imports.
- Some items going from private to pub(crate).

---------

Co-authored-by: Daniel McNab <[email protected]>

Author: PoignardAzur

examples/vello_editor/src/text.rs

@@ -8,7 +8,9 @@ use web_time::Instant;
 
 use accesskit::{Node, TreeUpdate};
 use core::default::Default;
-use parley::{GenericFamily, StyleProperty, editor::SplitString, layout::PositionedLayoutItem};
+use parley::editing::SplitString;
+use parley::layout::PositionedLayoutItem;
+use parley::{GenericFamily, StyleProperty};
 use std::time::Duration;
 use ui_events::pointer::PointerButton;
 use ui_events::{
@@ -23,7 +25,7 @@ use vello::{
 };
 use winit::event::{Ime, WindowEvent};
 
-pub use parley::layout::editor::Generation;
+pub use parley::editing::Generation;
 use parley::{FontContext, LayoutContext, PlainEditor, PlainEditorDriver};
 
 use crate::access_ids::next_node_id;