feat: warn for external dependencies

[pi0]

🆒 Your use case

Being aware if a dependency is missing and being inlined

🆕 The solution you'd like

Have console warning about externals and a config to mark them as externals instead of silently inlining

[pi0]

Silently inlining dependencies is becoming critical which I think worth trying to bring from unbuild (https://github.com/unjs/unbuild/blob/5004f8077995075c7e547a69ad3d89cc577cad3f/src/builder/rollup.ts#L105) (last time happened on nuxt image module)