add JSONText methods for Symbol and JSONText

[hhaensel]

In some contexts it would be beneficial to have JSONText constructors for Symbol and JSONText arguments. What do you think about the following defintions?

JSON.JSONText(js::Symbol) = JSONText(String(js))
JSON.JSONText(js::JSONText) = js

I think it doesn't hurt and only adds methods that a user would expect to work as defined.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.27%. Comparing base (f4fbb5a) to head (cbdc049).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #427      +/-   ##
==========================================
+ Coverage   90.26%   90.27%   +0.01%     
==========================================
  Files           7        7              
  Lines        1366     1368       +2     
==========================================
+ Hits         1233     1235       +2     
  Misses        133      133              

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