Rename nargo contract command

[TomAFrench]

As we add Noir contracts, there's going to become confusion around the nargo contract command. Does it create a solidity verifier contract or does it do something else related to noir contracts?

We should then update the name to avoid any confusion. One option is:

nargo generate-verifier --solidity perhaps? We could then extend it to other languages, etc.

Originally posted by @TomAFrench in #944 (comment)

[kevaundray]

Think we want to get this in quite quickly as its an annoying breaking change that is in a lot of places.

$ nargo generate-verifier --solidity

@noir-lang/tooling what do you think about the above change? Any suggestions for something better?

[joss-aztec]

I like your suggestion