Build command should emit typings

**Is your feature request related to a problem? Please describe.**
No typings files are emitted as an effect of running the build script. When installing Imprest inside of another typescript project there is subsequently no typing information available.

**Describe the solution you'd like**
The typescript CLI has a flag called `--emitDeclarationOnly`. This should be run as part of the build script to generate typings artefacts alongside the transpiled files inside of the `lib` directory.

**Describe alternatives you've considered**
Something more integrated with the babel command would have been nice but this does not exist.

Inspect the build script and its existing TypeScript or Babel commands, then check how output is placed in the lib directory. Run the build to confirm the current artifacts, add declaration-only output as requested, and verify that typings files appear alongside the transpiled files in lib.

Written by the indexing model from the issue text.