Fix selection{Start,End} when selectionDirection is "backward"

Per the spec, selectionStart and selectionEnd should return the same
values regardless of the selectionDirection. (That is, selectionStart is
always less than or equal to selectionEnd; the direction then implies
which of selectionStart or selectionEnd is the cursor position.)

There was no explicit WPT test for this, so I added one.

This bug was initially quite hard to wrap my head around, and I think
part of the problem is the code in TextInput. Therefore, in the process
of fixing it I have refactored the implementation of TextInput:

* Rename selection_begin to selection_origin. This value doesn't
  necessarily correspond directly to the selectionStart DOM value - in
  the case of a backward selection, it corresponds to selectionEnd.
  I feel that "origin" doesn't imply a specific ordering as strongly as
  "begin" (or "start" for that matter) does.

* In various other cases where "begin" is used as a synonym for "start",
  just use "start" for consistency.

* Implement selection_start() and selection_end() methods (and their
  _offset() variants) which directly correspond to their DOM
  equivalents.

* Rename other related methods to make them less wordy and more
  consistent / intention-revealing.

* Add assertions to assert_ok_selection() to ensure that our assumptions
  about the ordering of selection_origin and edit_point are met. This
  then revealed a bug in adjust_selection_for_horizontal_change() where
  the value of selection_direction was not maintained correctly (causing
  a unit test failure when the new assertion failed).

tests/wpt/web-platform-tests/html/semantics/forms/textfieldselection/selection-start-end.html

@@ -143,4 +143,30 @@
       el.remove();
     }
   }, "selectionEnd edge-case values");
+
+  test(() => {
+    for (let el of createTestElements(testValue)) {
+      const start = 1;
+      const end = testValue.length - 1;
+
+      el.setSelectionRange(start, end);
+
+      assert_equals(el.selectionStart, start, `selectionStart on ${el.id}`);
+      assert_equals(el.selectionEnd, end, `selectionEnd on ${el.id}`);
+
+      el.selectionDirection = "backward";
+
+      assert_equals(el.selectionStart, start,
+                    `selectionStart on ${el.id} after setting selectionDirection to "backward"`);
+      assert_equals(el.selectionEnd, end,
+                    `selectionEnd on ${el.id} after setting selectionDirection to "backward"`);
+
+      el.selectionDirection = "forward";
+
+      assert_equals(el.selectionStart, start,
+                    `selectionStart on ${el.id} after setting selectionDirection to "forward"`);
+      assert_equals(el.selectionEnd, end,
+                    `selectionEnd on ${el.id} after setting selectionDirection to "forward"`);
+    }
+  }, "selectionStart and selectionEnd should remain the same when selectionDirection is changed");
 </script>