(re)Add support for ast only file

With https://github.com/crytic/slither/pull/823, we removed the support for AST-only file in Slither.

We could re-add this support by adding a ast-only platform in crytic-compile.

Ideally the contract is compiled with `abi,ast,bin,bin-runtime,srcmap,srcmap-runtime,userdoc,devdoc,hashes,compact-format`, but we could allow a best-effort approach here.

The main difficulty that I can see here is that Slither now relies more on the source code being accessible, so we might need to disable some analyses in Slither.

Related https://github.com/crytic/slither/issues/864