Added Issues Closed / Opened ratio in Issue trends

[amunz]

Please consider my contribution to add the metric issues closed / issues opened ratio. The metric is defined by the Chaoss project as an important metric for determining growth maturity and decline. More information on the metric can be found here.

Thank you for considering, Adam

[Humbedooh]

Hi Adam, this looks great. Can you snake-case it please?

[michalslowikowski00]

Hi folks. Please take a look #45. We are going to change the master branch name to main