Adds 'report' output for ion schema validate

[popematt]

Issue #, if available:

None

Description of changes:

• Adds a module with ansi code constants that can be imported and used in a format!() string. See validate.rs for example usage.
• Adds more help documentation to the validate command.
• Fixes a clippy warning in complaint.rs.
• Adds a new "report" output format for the validate command. "Report" is a rather vague name, so if you have a better idea, please share it with me.

Example usage and output (but ok is green and FAILED is red, just like the cargo test report).

❯ cargo run -- schema -X validate -R 'document' **/*.ion

code-gen-projects/input/good/scalar/empty_value.ion ... ok
code-gen-projects/input/good/scalar/valid_value.ion ... ok
code-gen-projects/input/good/sequence/empty_sequence.ion ... FAILED
code-gen-projects/input/good/sequence/valid_elements.ion ... ok
code-gen-projects/input/good/struct_with_fields/empty_values.ion ... FAILED
code-gen-projects/input/good/struct_with_fields/valid_fields.ion ... ok
code-gen-projects/input/good/struct_with_fields/valid_unordered_fields.ion ... ok

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