[Hotfix] Add support for latest foundry version

[anvacaru]

With the latest version, foundry does not include the ast field anymore in the generated contract a artifacts by default (source).
This change makes kontrol build to fail because we look for the ast fields to read Natspec comments.

In this PR :

• add the ast = true config variable in the foundry.toml file to add the ast field
• add a descriptive error message if the field is not present.

Later, we can automatically add the --ast flag to the internal 'forge build' command. A version check would be needed, as this flag is not backwards compatible.