New handlebars helpers - jsonParse and jsonPath

We’ve added handlebars helpers to seamlessly handle JSON strings in the template editor:

• jsonParse - Converts a JSON string into an object, making it easier to apply conditions or use JSON strings in merge tags.
• jsonPath - Fetch data corresponding to a path within a JSON object. Works well with jsonParse to directly access nested data in JSON string without block helpers.

Fixes and Improvements:

• Opened up merge tag input to support handlebars helper in email merge tags.
• Added support for handlebars helper in display condition.