Cleanup handling of surrounding word logic, fixing crash in editor::SelectAllMatches (#33353)

This reduces code complexity and avoids unnecessary roundtripping
through `DisplayPoint`. Hopefully this doesn't cause behavior changes,
but has one known behavior improvement:

`clip_at_line_ends` logic caused `is_inside_word` to return false when
on a word at the end of the line. In vim mode, this caused
`select_all_matches` to not select words at the end of lines, and in
some cases crashes due to not finding any selections.

Closes #29823

Release Notes:

- N/A

crates/editor/src/editor_tests.rs

@@ -6667,6 +6667,15 @@ async fn test_select_all_matches(cx: &mut TestAppContext) {
     cx.update_editor(|e, window, cx| e.select_all_matches(&SelectAllMatches, window, cx))
         .unwrap();
     cx.assert_editor_state("abc\n«  ˇ»abc\nabc");
+
+    // Test with a single word and clip_at_line_ends=true (#29823)
+    cx.set_state("aˇbc");
+    cx.update_editor(|e, window, cx| {
+        e.set_clip_at_line_ends(true, cx);
+        e.select_all_matches(&SelectAllMatches, window, cx).unwrap();
+        e.set_clip_at_line_ends(false, cx);
+    });
+    cx.assert_editor_state("«abcˇ»");
 }
 
 #[gpui::test]