build related dialects in parallel

[isaacs]

Of course, CommonJS and ESM can't be built in parallel, since we have to swap out the package.json file that tells TSC what to build. But when there are multiple CommonJS or ESM dialects, they can be built in parallel, and it'd go a bit faster in cases where there are a lot of them.

[mpodwysocki]

@isaacs Would something like this work for example? https://gist.github.com/mpodwysocki/a26cc15f9ae04dc327d51fe73367e3dd

[isaacs]

@mpodwysocki It might! Just need to make sure it's not building into the same outDir.