Add validateAST command and/or validate on print

[DianaLease]

Context

Today, there is no easy way to check if a Concerto JSON AST is valid. When running concerto print, we get back a CTO string with errors. We should add a validateAST command and/or validate on print.

Expected Behavior

New concerto validate-ast command
and/or
concerto print should throw an error when given an invalid AST as input

Steps to Reproduce

1. Create an invalid JSON AST (eg. use invalid values for enum options)
2. Run concerto print on that AST
3. See invalid CTO as output

Existing issues

• Stack Overflow issues
• GitHub Issues

Context

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