[FEAT] Override externals

[schlusslicht]

What this pull request does
This pull request enables the consumer to configure the list of external rollup module IDs. Any value set to the external configuration property overrides the inherited rollup external option with the value provided.

What this pull request contains

• implementation
• documentation

Further notes
My use case is creating a library with an inlined webworker to be used by downstream projects on different platforms. To achieve this, the bundled webworker needs to contain all transpiled imports, as downstream bundlers will not be able to package additional dependencies into the blob. By setting the newly introduced property external: [], I can instruct rollup to consider no imports as external and bundle them into the worker blob. Which I can then savely distribute as functional part of my library.

Let me know what You think! 🎈

[darionco]

My first reaction when I saw this was that it could cause conflicts with the consuming code, on a second thought though, that shouldn't be a problem since workers run on isolated contexts. Maybe if the main thread and workers pass objects around from incompatible versions of a library, but that is a corner case and I think it would be reasonable to require consumers to make sure library versions are compatible.

I couldn't think of a better solution to this specific problem without requiring consumers to use rollup and this plugin (or equivalent tooling) which would be inconvenient. It is worth noting that using this functionality will increase payload size, yet again, can't think of a better solution.

I'll merge this and release a new version. Thank you for your contribution!

[darionco]

Released in version 1.5.0