title: JSON_MODIFY appends variable containing JSON with escape characters instead of JSON string
body: <p>I have had some success using <code>JSON_MODIFY</code> to append <code>fieldErrors: []</code> and its content to the root of another object <code>{ data: []}</code> so that the resultant object looks something like this:  <code>{data: [], fieldErrors: []}</code>.</p>
  
  <p>Problem is, when I append a @variable, it includes a bunch of escape characters.</p>
  
  <p>I expect this:  <code>{"name":"PosTitle","status":"the field is messed up, yo"}</code></p>
  
  <p>BUT, I get this:  <code>["{\"name\":\"PosTitle\",\"status\":\"the field is messed up, yo\"}"]}</code></p>
  
  <pre><code>    DECLARE
          @fieldErrors nvarchar(max) ='{}'
          ,@jsonResponse nvarchar(max) = '
      {
          "data": [
              {
                  "PosTitle": "",
                  "PosCode": "86753",
              }
          ]
      }
      '
  
      --define the fields that are bad
      set @fieldErrors = JSON_MODIFY(JSON_MODIFY(@fieldErrors, '$.name', 'PosTitle'), '$.status', 'the field is messed up, yo')
      print @fieldErrors
  
      --RESULT, this looks great:
      --{"name":"PosTitle","status":"the field is messed up, yo"}
  
  
  
      -- append fieldErrors to the response
  
      set @jsonResponse = JSON_MODIFY(@jsonResponse, 'append $.fieldErrors', @fieldErrors)
      print @jsonResponse
  
      --RESPONSE, this includes escape characters
  /*
      {
          "data": [
              {
                  "PosTitle": "",
                  "PosCode": "86753",
  
              }
          ]
      ,"fieldErrors":["{\"name\":\"PosTitle\",\"status\":\"the field is messed up, yo\"}"]}
  
  
  */
  </code></pre>
  
  <p>Why are escape characters being added when fieldErrors is appended to the response?</p>