Gemfile: Add tsort, for rbs, on Ruby 4.1.0+ to avoid LoadError

[olleolleolle]

In order to avoid decpreation warnings when running rbs on newer versions of Ruby.

/home/runner/work/webrick/webrick/vendor/bundle/ruby/4.1.0+1/gems/rbs-3.10.2/lib/rbs.rb:11: warning: tsort used to be loaded from the standard library, but is not part of the default gems since Ruby 4.1.0.
You can add tsort to your Gemfile or gemspec to fix this error.

EDIT: Another solution could be "fix and release a new rbs which does this".

[jeremyevans]

This change seems odd to me. I'm not against it, but I would think that the correct fix is for rbs to add a dependency on tsort, so gems using rbs don't need an explicit dependency like this on tsort. Otherwise, everything using rbs would have to have a similar change, and that seems undesirable. Better to fix the issue in one place than many places. However, maybe there is a reason that would not work?

[olleolleolle]

@jeremyevans No reason, rbs to fix the issue is the next and logical step. This PR can be reverted after such a change has been made. I poked someone in the rbs project that had an open PR about tsort.