feat(Aztec.nr): Remove the `internal` keyword, in favour of an attribute

[LHerskind]

Right now the keyword internal is used in function definitions to indicate that the function is to only be called by the contract itself. The keyword is only useful in contract noir, and is merely setting a flag in the abi which is later used to generate the function tree root.

#[aztec(public)]
open internal fn some_function() {
    // ...
}

Should become:

#[aztec(public,internal)]
open fn some_function() {
    // ...
}

While maintaining its functionality.

[LHerskind]

This was addressed with #4898