Lower log level of warnings in ansi.rs
Lower log level of warnings in ansi.rs

Since ansi.rs is mostly about control sequences sent by applications,
displaying all issues during parsing to the user can be annoying since
Alacritty might not actually do anything wrong.

To resolve this problem, all `warn!` logs in `src/ansi.rs` have been
decreased to the `debug!` level.

This fixes #1809.
alacritty/alacritty,jwilm/alacritty,alacritty/alacritty,jwilm/alacritty,jwilm/alacritty,jwilm/alacritty