fix: include sourcemap files in the dist

[prashantpalikhe]

Fixes #831

[fb55]

This will lead to the next issue, which is that source maps reference source files (which aren't part of the package). I've solved this in most of my modules by specifying a sourceRoot that points at the source files on GitHub. That won't work for parse5 however, as we are using tsc --build to build a number of packages at once, and --build does not allow sourceRoot to be set.

The best way forward here is probably to disable source maps during manual builds, or how that the TypeScript team will add support for sourceRoot in --build.