demo_gen: Add support for custom demonstration functions

[ambiguousname]

Don't have time to do this, but here's what I'd do:

Add to DemoInfo in core/hir/attrs.rs, specifically a custom_func() attribute, where it:

1. Points to a Javascript file.
2. Lists all of the primitive inputs that the function takes.

Then under demo_gen/mod.rs, for every function you find with that attribute, copy the Javascript file to the output folder. Then add the function and parameter information to RenderInfo

This would be really nice to set up the Segmenter in ICU4X.