Now the formatter configuration is per Ractor. DefaultFormatter is used
if not set.
DefaultFormatter#message_for is now a class method to allow sub-Ractors
to call the method.
https://github.com/ruby/error_highlight/commit/9fbaa8ab7c
Now, the highlight line is created by replacing non-tab characters with
spaces, and keeping all hard tabs as-is. This means the highlight line
has the completely same indentation as the code snippet line.
Fixes#7https://github.com/ruby/error_highlight/commit/38f20fa542
