Cleanup dist/ in build task

The TypeScript compiler does not clean up the `outDir` before compiling, so when files are renamed or moved, the old files remain in `dist`. It's recommended to add a clean task to the build script (see [this issue](https://github.com/Microsoft/TypeScript/issues/13722)).

Start by locating the build script and how the TypeScript compiler writes to dist through outDir. Verify that renamed or moved source files leave stale output behind, then add the cleanup step and confirm a build removes obsolete files from dist.

Written by the indexing model from the issue text.