Create a Pharo13 branch

[astares]

Hi Alain @plantec

I think it would make sense for Toplo (beside the existing "Pharo12" branch) to now also create a "Pharo13" branch.

This would follow the pattern we have for other external projects (who already provide such a branch) like

• https://github.com/pharo-spec/ScriptableDebugger/tree/Pharo13
• https://github.com/pharo-spec/NewTools/tree/Pharo13
• https://github.com/pharo-spec/Spec/tree/Pharo13
• ...

Once done we can adopt BaselineOfPharo class>>#toplo in Pharo 13 to reflect it.