Proposal: add support for fast transpilation tools under `isolatedDeclarations`

[DaniGuardiola]

We briefly discussed fast transpilation tools in #31, and with isolatedDeclarations I think it is worth revisiting. I know this option and its surrounding features and ecosystem are still in their infancy. However, I don't think it'll be long before it matures, and we can both use tsc faster (for both building and generation of declaration files, without the slow checker running at any point) and also third-party fast tools (again, for both). It might even make sense to use a combination of third-party tools for transpilation + tsc for declaration files.

I'm just filing this to get the conversation started :)

[isaacs]

I think the thing to do is to add a buildCommand config, as suggested in #31 (comment)

Then we could experiment with whatever, but set the default to something sensible, and be able to play around with other options.

[DaniGuardiola]

That sounds good to me, and I'd be happy to help test it out.