Add a lint for missing `no_main`

[ascjones]

Since use-ink/cargo-contract#1076 we require contracts to be annotated with a no_mainouter attribute. We added a feature in cargo_contract to check for the error use-ink/cargo-contract#1113, but it involves some brittle code to intercept and redirect the stderr output of cargo-contract in order to catch the specific error.

If it is possible to check for attributes, it would be preferable to do this as a lint.