Add issue_tracker link for pub.dev

[nohli]

This PR adds a link to GitHub issues to pub.dev (in the right column right under the repository link).

[codecov[bot]]

Codecov Report

Merging #778 (dd3fbeb) into master (e937b72) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #778   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          12       12           
  Lines         728      728           
=======================================
  Hits          723      723           
  Misses          5        5           

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[rrousselGit]

Thanks!