feat: always provide `Default` for `MultiCompiler`

[klkvr]

Closes https://github.com/foundry-rs/compilers/issues/187 Closes https://github.com/foundry-rs/compilers/issues/168

Updates MultiCompiler and makes solc compiler on it optional. If svm-solc is not activated, it is attempted to initialize solc via Solc::new("solc"), and if solc binary is not available in command line, it is set to None.

We could also add svm-solc to default features as I think people would often prefer enabling it as it's required to emulate foundry's default behavior.