text: Implement double/triple click selection

[kjarosh]

Fixes https://github.com/ruffle-rs/ruffle/issues/14996, progresses https://github.com/ruffle-rs/ruffle/issues/1891.

This PR implements selecting words by double-clicking and selecting lines by triple-clicking. It also includes support for selecting words and lines while dragging.

A test is added to verify this behavior. Example use cases covered by this test:

1. selecting characters,
2. selecting words by double-clicking,
3. selecting multiple words by double-clicking and dragging,
4. selecting lines by triple-clicking,
5. selecting multiple lines by triple-clicking and dragging,
6. double-clicking at word boundaries,
7. double-clicking between spaces,
8. changing the underlying text while clicking and dragging,
9. trying to select while the text field is not selectable.