[Feature] Show Variable Substitution in Debug

[b1scuit-thi3f]

Is your feature request related to a problem? Please describe.
When debugging flows it is sometimes difficult to understand what has been substituted where. For example, if an action fails it may be due to an incorrect reference (e.g. the field is set to dta[var][src_ip] which isn't valid). However this might not be obvious unless one reads over all the fields.

Describe the solution you'd like
Debug should keep track of all variables being substituted and allow the user to visually see changes - either via a popup on hover or in the data-out box.