Fixes #55704

The `escape` function in `crates/markdown/src/markdown.rs` was calling
`c as u8` on the `char`s before passing to `MarkdownEscaper::next()`.
This strips non ASCII Unicode codepoints down to just their low 8 bits
which might be in the ASCII punctuation range and thus cause an extra
backslash to be added in front of these non ASCII chars.
 
Release Notes:

- Fixed a bug where non-ASCII chars in diagnostic messages were
incorrectly rendered with spurious `\` characters