Parser constraints

[vncsmyrnk]

Validate and enforce expected node structure and returns errors on detected mismatches.

[vncsmyrnk]

Maybe set this validations at AST level?

[vncsmyrnk]

Maybe set this validations at AST level?

Constraints and invalid nodes are parser-level validations. Maybe structural validations (e.g. subcommands and flags/arguments shouldn't be allowed) are more AST-level than parser-level.