Link the Q&A tab on the marketplace to SO

[brettcannon]

... as documented at https://code.visualstudio.com/docs/extensionAPI/extension-manifest

Fixes #1141

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5416af2). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1157   +/-   ##
=========================================
  Coverage          ?   72.13%           
=========================================
  Files             ?      260           
  Lines             ?    11898           
  Branches          ?     2112           
=========================================
  Hits              ?     8583           
  Misses            ?     3192           
  Partials          ?      123

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