`deno fmt --check` was broken for CSS, YAML and HTML files.
Before this PR, formatting any of these file types would return a
string, even though the contract in `cli/tools/fmt.rs` is to only return a
string if the formatting changed. This causes wrong flagging of these files
as being badly formatted even though diffs showed nothing (because
they were in fact formatted properly).
Closes https://github.com/denoland/deno/issues/25840
