feat: catch JSON stringified

[baruchiro]

Description:

I have a code sample when someone writting a hard-coded JSON as string, and it looks like this:

var json = "{ \"access-key\": \"access key here\" }"

Checklist:

• Does your PR pass tests?
• Have you written new tests for your changes?
• Have you lint your code locally prior to submission?

Original: gitleaks#1267