Add js-options field so that user-defined options can be passed to javascript preprocessor

[Swordlash]

Describe the feature request
See the GHC ticket and description of the issue: https://gitlab.haskell.org/ghc/ghc/-/issues/25632#note_602729

There seems to be currently no way to pass JS CPP options for js-sources, like defining your own macros.

Additional context
This would enable things like TRACE flags in libraries that conditionally compile logging directives, for instance.

[Mikolaj]

Is this a newcomer ticket (if so, let's label it so), or can there be anything tricky, either in implementation or testing (I guess no testing is possible at all with our current CI?)?

[Swordlash]

@Mikolaj Looks easy enough, can you assign me?