feat: Adding internal keyword

[LHerskind]

Description

Adding internal as a keyword for functions to specify that only the contracts itself can execute this function.

Related to #1168. But functions are not internal by default, instead it is callable by other contracts and itself.

Problem*

When building noir contracts for Aztec, we want to support flows such as:

• Private functions does some access check and calls a public function to mint tokens

Currently, we could do this by inserting a commitment from private and then nullify it in public, but that is very wasteful.

Instead we can introduce internal as a keyword and store is_internal as part of the contracts tree, which allows the kernel to check that the contract itself is the caller, and reject anyone else from calling it.

Similar to solidity internal.

Summary*

• Adds internal keyword.
• Adds is_internal to json output

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.

For documentation only of interest to contract noir, which don't seem to have documentation at the moment.

Additional Context

PR Checklist*

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

[jfecher]

Looks good, sans one more parser comment.

Since this is still a draft, what work remains on this?

[LHerskind]

Looks good, sans one more parser comment.

Since this is still a draft, what work remains on this?

The idea was that it should be behind a feature flag to see how easily we could use those to get things into noir for Aztec without wrecking master, just have not gotten around to it.