Implement JSONX.JSONText

[JamesWrigley]

This would be needed by IJulia.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.92%. Comparing base (a2aa9ed) to head (76f3775).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #394   +/-   ##
=======================================
  Coverage   89.92%   89.92%           
=======================================
  Files           7        7           
  Lines        1320     1320           
=======================================
  Hits         1187     1187           
  Misses        133      133           

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[quinnj]

Thanks!