Add support for populating multiple symbols nested inside another symbol
**Which version of the plugin are you using?**
[X] Data Populator for Sketch
[ ] Data Populator for Adobe XD

**Is your feature request related to a problem? Please describe.**
Currently, you can populate individual symbols and their respective {placeholder} text fields and images with no issues, but if you nest those individual symbols into another symbol, you can no longer populate them individually. Instead, running the "Populate with JSON" command on that group symbol will populate all of the nested symbols with the same data.

**Describe the solution you'd like**
Allow a user to uniquely populate nested symbols.

**Additional context**
Video to illustrate the issue:
https://user-images.githubusercontent.com/22907720/123016061-a73c0280-d37e-11eb-88b8-f5d568cb792a.mov
(Sorry for the low-res. Silly Github file size constraints.)
