chore: replace big_text with hardcoded logo (#1203)

big_text.rs was a copy of the code from tui-big-text and was getting
gradually out of sync with the original crate. It was also rendering
something a bit different than the Ratatui logo. This commit replaces
the big_text.rs file with a much smaller string representation of the
Ratatui logo.

![demo2](https://raw.githubusercontent.com/ratatui-org/ratatui/images/examples/demo2-destroy.gif)