Extract common text control selection code

The API for text control selection is the same for both <input> and
<textarea>:

https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#textFieldSelection

Before this change, they had similar but not identical implementations
with duplicate code. Now there is a common TextControl trait which
contains the implementation used by both. As a result, some previously
failing tests now pass.

components/script/dom/mod.rs

@@ -439,6 +439,7 @@ pub mod testrunner;
 pub mod testworklet;
 pub mod testworkletglobalscope;
 pub mod text;
+pub mod textcontrol;
 pub mod textdecoder;
 pub mod textencoder;
 pub mod touch;