Use $stderr instead of STDERR for Ractor

[wanabe]

Hello,

I found that ErrorHighlight::DefaultFormatter.terminal_width can't be called from non-main Ractor.

$ ruby -Ilib -rerror_highlight -e 'Ractor.new { p ErrorHighlight.formatter.terminal_width }.take'
-e:1: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
#<Thread:0x00007fcde7022b98 run> terminated with exception (report_on_exception is true):
/home/wanabe/work/prog/ruby/error_highlight/lib/error_highlight/formatter.rb:61:in 'ErrorHighlight::DefaultFormatter.terminal_width': can not access non-shareable objects in constant ErrorHighlight::DefaultFormatter::STDERR by non-main Ractor. (Ractor::IsolationError)
        from -e:1:in 'block in <main>'
<internal:ractor>:711:in 'Ractor#take': thrown by remote Ractor. (Ractor::RemoteError)
        from -e:1:in '<main>'
/home/wanabe/work/prog/ruby/error_highlight/lib/error_highlight/formatter.rb:61:in 'ErrorHighlight::DefaultFormatter.terminal_width': can not access non-shareable objects in constant ErrorHighlight::DefaultFormatter::STDERR by non-main Ractor. (Ractor::IsolationError)
        from -e:1:in 'block in <main>'

Replacing STDERR to $stderr resolves this issue because $stderr is a ractor-local global variable.

$ ruby -Ilib -rerror_highlight -e 'Ractor.new { p ErrorHighlight.formatter.terminal_width }.take'
-e:1: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
104

How do you like it? Ijust want to solve the issue, so the method can be anything.

[mame]

Thanks, it looks like the Ruby interpreter writes uncaught exception messages to $stderr instead of STDERR

$ ruby -e '$stderr = open(IO::NULL, "w"); 1.time'

$ ruby -e 'STDERR = open(IO::NULL, "w"); 1.time'
-e:1: warning: already initialized constant STDERR
-e:1:in '<main>': undefined method 'time' for an instance of Integer (NoMethodError)
Did you mean?  times

So it seems correct for error_highlight to use $stderr here.