Add thousands separator to Views result summary

This issue is being reviewed by the kind folks in Slack, #needs-review-queue-initiative. We are working to keep the size of Needs Review queue [2700+ issues] to around 400 (1 month or less), following Review a patch or merge request → as a guide.

For the new configuration

+    thousand_separator:
+      type: text
+      label: 'Thousand marker'

Will need a update hook to update views using that block. So users aren't surprised later if they just save the view.
Will need a test for the update hook too.

Everything else looked good.

Thanks!

the patch from #20 wasn't applying to D10.3, I manually applied and created a fork for the issue.