Support bundling of JS snippets

[sliemeobn]

as mentioned in #477 we currently have no good solution for creating a library that contains both Swift and JS code.

even if we can automatically "import" JS globals via BridgeJS, certain cases will require JS code, eg when:

• you need JS features that are not exposed through the bridge (eg: create a class)
• the back and forth through the bridging layer adds unnecessary runtime work for a "chunk of JS work"

here is a use case from ElementaryUI:
I want to create Swift API that let's you define custom elements ("web components").
it is currently incubated in the elementary.codes repo (ts code, swift code)

ideally we can just develop a Swift package with JS functions and Swift functions, add and attribute, and it will all "just work" in the final app that uses this library.