Improved text file identification to handle json files

[denis256]

Description

Included changes:

• updated text file identification to handle JSON files
• added integration tests to track that variables in JSON files are replaced

Fixes #168.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added / Removed / Updated [X].

Updated JSON file processing to substitute placeholder variables.

Migration Guide

[brikis98]

LGTM!