feat: Add pub modifier

[jfecher]

Description

Problem*

Resolves #776

Summary*

Superceeds PR #2513

Adds the pub modifier as in #2513, but refactors the PR to check within name resolution rather than adding a separate pass to check for visibility.

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.
    • We will need to mention functions can be made public now pub fn foo(){}
    • Functions that are not public are now no longer visible from outside the crate they were defined in, or from any parent module within the same crate.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[kevaundray]

@guipublic assigning you as a reviewer

[kevaundray]

Can merge after merge conflicts are resolved!

[Savio-Sou]

Would this be a breaking change?

As in all existing Noir functions before this PR without the pub modifier are treated as public by default, but after this PR they would be treated as private by default?

[guipublic]

Yes, functions are now treated as private by default, you need the pub modifier to make them public.
BUT:

• it is for now a warning, so I is not a breaking change. You are notified that this will break in the future
• it is only for librairies. If you use function from your own module, it will not complain.
• noir std library has the pub modifier.