[Feature] Do not allow the 'constructor' keyword

[vicsn]

🚀 Feature

We should no longer allow the 'constructor' keyword to be in line with: ProvableHQ/snarkVM#2685

We can also explore whether it makes sense to add that check somewhere in snarkVM (where Leo calls into, so the result is the same: users are not encouraged to use this keyword anymore).

[tetsuo-cpp]

I'll give this a try if that's ok.