script: Add a Rope type and use it for TextInput (#41650)

`TextInput` internally stores a rope of text content, one segment for
each line. This change separates out the rope into a separate `Rope`
data structure that can be shared with other parts of Servo.

`TextInput` needs to move through text content in a variety of ways,
depending on the keys pressed when interacting with a text area. This
change provides a unified movement API in both `Rope` and in
`TextInput` ensuring that selection is handled consistently (apart from
a few minor quirks [^1]). This simplifies the code an improves
interactive behavior.

[^1]: One of these quirks is that the edit point will move to
directional end of the motion when collapsing a selection, but only when
moving by grapheme or word (and not by line). These quirks existed in an
undocumented way previously and they are preserved with code comments.

Testing: This is covered by existing unit tests (updated for the new
API) and
the WPT suite.

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

components/script/dom/html/htmltextareaelement.rs

@@ -6,6 +6,7 @@ use std::cell::Cell;
 use std::default::Default;
 use std::ops::Range;
 
+use base::Lines;
 use base::text::{Utf8CodeUnitLength, Utf16CodeUnitLength};
 use dom_struct::dom_struct;
 use embedder_traits::{EmbedderControlRequest, InputMethodRequest, InputMethodType};
@@ -47,7 +48,7 @@ use crate::dom::validitystate::{ValidationFlags, ValidityState};
 use crate::dom::virtualmethods::VirtualMethods;
 use crate::script_runtime::CanGc;
 use crate::textinput::{
-    ClipboardEventFlags, Direction, IsComposing, KeyReaction, Lines, SelectionDirection, TextInput,
+    ClipboardEventFlags, IsComposing, KeyReaction, SelectionDirection, TextInput,
 };
 
 #[dom_struct]
@@ -383,7 +384,7 @@ impl HTMLTextAreaElementMethods<crate::DomTypeHolder> for HTMLTextAreaElement {
 
             if old_value != textinput.get_content() {
                 // Step 4
-                textinput.clear_selection_to_limit(Direction::Forward);
+                textinput.clear_selection_to_end();
             }
         }