feat: Implement 'open' and 'unconstrained' keywords

[jfecher]

Related issue(s)

Resolves #(none)

Description

Summary of changes

Adds the 'open' and 'unconstrained' keywords for contract visibility. These keywords are mutually exclusive and functions with neither keyword are considered to be 'secret' by default. It is an error for functions outside of a contract to specify that they are open or unconstrained (though we may allow unconstrained functions in the future).

These keywords currently do nothing until #1033 is merged, when they will only set the is_secret option in the resulting JSON after compiling a contract.

Dependency additions / changes

None

Test additions / changes

None

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Documentation needs

• This PR requires documentation updates when merged.

Once we document contracts, we will need to document that they can be secret (the default, no keyword), open, or unconstrained.

Additional context

[vezenovm]

Just one nit