add a CLI tool to convert MVT->MLT (and back?) for testing and other usecases

[nyurik]

In order to simplify integration testing, it would be good to implement a simple CLI tool that converts a single file from one format to another. This is a better path than to implement integration testing in code - something that runs through all code and compares result.

Benefits:

• it can be used by an individual for a single file
• it can be used by CI the same way on a single file or all files
• it can be easily parallelized by CI using bash tools or other ways
• the output result can be stored more easily in the CI-controlled dir to automate comparison
• the output can be passed to another utility, possibly written in a different language, to parse it into some textual representation
• text representation can be stored in GIT to aid with PR reviews / change evaluation
• any conversion failure (unexpected output) will not be a simple "result does not match", but instead can be printed as a text diff