Fixes issue with inserting content with height>viewport_area.height and adds
the ability to insert content of height>terminal_height
- Adds TestBackend::append_lines() and TestBackend::clear_region() methods to
support testing the changes
Windows cannot compile termion, so it is not included in the docs.
Rustdoc will fail if it cannot find a link, so the docs fail to build
on windows.
This replaces the link to TermionBackend with one that does not fail
during checks.
Fixes https://github.com/ratatui-org/ratatui/issues/498
